The globules take no share in the process of nutrition, it cannot be doubted that they play a part in the process of respiration. The compound of iron in the globules has the characters of an oxidized compound; for it is decomposed by sulphuretted hydrogen, exactly in the same way as the oxides or other analogous compounds of iron. By means of diluted mineral acids, peroxide (sesqui-oxide) of iron may be extracted, at the ordinary temperature, from the fresh or dried red colouring matter of the blood. The characters of the compounds of iron may perhaps assist us to explain the share which that metal takes in the respiratory process. No other metal can be compared with iron for the remarkable properties of its compounds. The compounds of protoxide of iron possess the property of depriving other oxidized compounds of oxygen; while the compounds of peroxide of iron, under other circumstances, give us oxygen with the uttermost facility. Hydrated peroxide of iron, in contact with organic matters destitute of sulphur, is converted into carbonate of the protoxide. Carbonate of protoxide of iron, in contact with water and oxygen, is decomposed; all the carbonic acid is given off, and by absorption of oxygen it passes into the hydrated peroxide, which may again be converted into a compound of the protoxide. Not only the oxides of iron, but also the cyanides of that metal, exhibit similar properties. Prussian blue contains iron in combination with all the organic elements of the body; hydrogen and oxygen (water), carbon and nitrogen (cyanogen). When it is exposed to light, cyanogen is given off, and it becomes white; in the dark it attracts oxygen, and recovers its blue colour. All these observations, taken together, lead to the opinion that the globules of arterial blood contain a compound of iron saturated with oxygen, which in the living blood loses its oxygen during its passage through the capillaries. The same thing occurs when it is separated from the body and begins to undergo decomposition. The compound, rich in oxygen, passes therefore, by the loss of oxygen, into one far less charged with that element. One of the products of oxidation formed in this process is carbonic acid. The compound of iron in the venous blood possesses the property of Combining with carbonic acid; and it is obvious that the globules of the arterial blood, after losing a part of their oxygen, will, if they meet with carbonic acid, combine with that substance. When they reach the lungs, they will again take up the oxygen they have lost; for every volume of oxygen absorbed, a corresponding volume of carbonic acid will be separated; they will return to their former state, that is, they will again acquire the power of giving off oxygen. For every volume of oxygen which the globules can give off, there will be formed (as carbonic acid contains its own volume of oxygen without condensation) neither more nor less than an equal volume of carbonic acid. For every volume of oxygen which the globules are capable of absorbing, no more carbonic acid can possibly be separated than that volume of oxygen can produce. When carbonate of protoxide of iron by the absorption of oxygen passes into the hydrated peroxide, there are given off, for every volume of oxygen necessary to the change from protoxide to peroxide of iron, four volumes of carbonic acid gas. But from the one volume of oxygen only one volume of carbonic acid gas can be produced. And the absorption of one volume of oxygen can only cause directly the separation of an equal volume of carbonic acid; consequently the substance or compound which has lost its oxygen during the passage of arterial into venous blood, must have been capable of absorbing or combining with carbonic acid; and we find, in point of fact, that the living blood is never, in any state, saturated with carbonic acid; that it is capable of taking up an additional quantity without any apparent disturbance of the functions of the globules. Thus, for instance, after drinking effervescing wines, beer, or mineral waters, more carbonic acid must necessarily be expired than at other times. In all cases where the oxygen of the arterial globules has been partly expended otherwise than in the formation of carbonic acid, the amount of this latter gas expired will correspond exactly with that which has been formed; less however will be given out after the use of fat and of still-wines than after champagne. Accord ing to the views now developed, the globules of arterial blood in their passage through the capillaries yield oxygen to certain constituents of the body. A small portion of this oxygen serves to produce the change of matter, and determines the separation of living parts, and their conversion into lifeless compounds, as well as the formation of the secretions and excretions. The greater part, however, of the oxygen is employed in converting into oxidized compounds the newlyformed substances which no longer form part of the living

tissues. In their return towards the heart, the globules whe have lost their oxygen combine with carbonic acid, producing venous blood; and when they reach the lungs, an exchan takes place between this carbonic acid and the oxygen of the atmosphere. The organic compound of iron, which exists in venous blood, recovers in the lungs the oxygen it has lost, and in consequence of this absorption of oxygen the carbane acid in combination with it is separated.'

Mulder is strongly opposed to this theory; he denies that the iron takes any essential part in the respiratory proom, and he refers the process entirely to the oxidation of the pr tein-compounds. He alleges the following grounds agains the probability of the correctness of Liebig's views:— 1. The iron is so intimately connected with the other ele ments of hæmatin, that it cannot be removed even by long gestion of this constituent in dilute hydrochloric or suljhare acid. If these re-agents cannot effect its oxidation, a a highly improbable that it should be oxidized in the lungs Respecting Liebig's assertion that dilute acids remove from dried blood, Mulder proves that this fact is valucien u relation to his theory, because other constituents of the blood besides the hæmatin contain this metal, apparently in a oxidized state.

2. If, as Liebig asserts, peroxide of iron exists in artera blood, and carbonate of protoxide of iron in venous blood, alwes any dilute acid would be capable of removing it. But this a not the case. Hæmatin, properly prepared, may be digested with dilute hydrochloric or sulphuric acid for many can without the least diminution in the quantity of the iron. From hæmatin treated in this manner Mulder obtained by cou bustion 9:49 per cent. of peroxide of iron, which is the comstant quantity always left after the combustion of well-prepared hæmatin. 3. The probability that the iron exists in a metallic state s strongly supported by the observation that hydrogen u evolved when a clot of blood is digested in sulphuric acid, and water is added. Mulder suggests that it occurs as an integral constituent of hæmatin in just the same manner that some occurs in sponge, sulphur in cystin, or arsenic in the cacodyl series.

4. The amount of hæmatin in the whole mass of the blood is far too inconsiderable to carry a due supply of oxygen to the whole system.

Having thus shown the principal objections to which Labig's celebrated theory is open, we shall endeavour brey to explain the rival theory of Mulder. We have at an early part of this article shown that the protein-compounds a capable of undergoing oxidation when in contact with the oxygen of the air. When a protein-compound becom oxidized, it assumes a plastic character, that is to say, it has a tendency to become solid and to adhere to solid substances. Now we have already mentioned that the blood-corpuscles are cells, of which the wall consists of a protein-compound named globulin. When a respiration is performed, the exterior layer of such of the corpuscles as are exposed in the lungs to the arian of the air, becomes converted into oxidized protein; it becomes whitish and less transparent. This is the state in which the corpuscles exist in arterial blood. As they reach the capa system, this white exterior layer is employed in the change material of the body, and is in that way consumed. Having bat this white layer, they again become transparent. The cart colouring substance in the corpuscles of arterial blood, suKA”Ş through a white layer, must necessarily appear of a bright reć tint, as may be shown by pouring dark red blood into a vend of milky glass.

The preceding observations have been made with the vice of showing the utility of these isolated animal cells-the bloodcorpuscles-in the respiratory process. (We shall revert agum to the distinctions between the characters of venous and rial corpuscles in our remarks on the colour of the blood.)

In our remarks on various tissues we shall often again have to notice the functions of isolated cells. We shall now g another illustration of their utility, namely, their importance in the process of nutrition. Mr. Goodsir has recently shouz that there is a continual development of cells at the extrem. Tr of each villus in the small intestine, and that these cells are the agents by which the secretion of the nutritions faid a accomplished, and by which it undergoes its first preparation for the purposes it is subsequently to fulfil. The preeen is so singular and interesting, that we give Mr. Gooder's observations in his own words, omitting those portions which do not bear specially on the point.

'As the chyle begins to pa's along the small inteséne, i

increased quantity of blood circulates in the capillaries of the gut. In consequence of this increased flow of blood, or from some other cause with which I am not yet acquainted, the internal surface of the gut throws off its epithelium, which is intermixed with the chyme in the cavity of the gut. The cast-off epithelium is of two kinds,-that which covers the villi, and which, from the duty it performs, may be named protective epithelium; and that which lines the follicles, and is endowed with secreting functions. The same action then, which in removing the protective epithelia from the villi prepares the latter for their peculiar function of absorption, throws out the secreting epithelia from the follicles, and thus conduces towards the performance of the function of these follicles. The villi, being now turgid with blood, erected, and naked, are covered or coated by the whitish-grey matter already described. This matter consists of chyme, of cast-off epithelia of the villi, and of the secreting epithelia of the follicles. The function of the villi now commences. The tninute vesicles which are interspersed among the terminal loops of the lacteals of the villus, increase in size by drawing materials from the blood through the coats of the capiliary ■ vessels, which ramify at this spot in great abundance. While this increase in their capacity is in progress, the growing vesicles are continually exerting their absorbing function, and draw into their cavities that portion of the chyme in the gut necessary to supply materials for the chyle. When the vesicles respectively attain in succession their specific size, they burst or dissolve, their contents being cast into the texture of the villus, as in the case of any other species of interstitial cell. The debris, and the contents of the dissolved chyle cells, as well as the other matters which have already subserved the nutrition of the villus, pass into the looped network of lacteals, which, like other lymphatics, are continually employed in this peculiar function. As long as the cavity of the gut contains chyme, the vesicles of the terminal extremity of the villi continue to develop, to absorb chyle, and to burst, and their remains and contents to be removed along the lacteals. When the gut contains no more chyme, the flow of blood to the mucous membrane diminishes, the development of new vesicles ceases, the lacteals empty themselves, and the villi become flaccid. The function of the villi now ceases till they are again roused into action by another flow of chyme along the gut. During the intervals of absorption, it becomes necessary to protect the villi from the matters contained in the bowel. They had thrown off their protective epithelium when required to perform their functions, just as the stomach had done to afford gastric juice, and the intestinal follicles to supply their peculiar secretions. In the intervals of digestion the epithelium is rapidly reproduced.'

Fig. 4.

Extremity of a villus with its absorbent vesicles distended with chyle, and the trunks of its lacteals seen through its coats. Very highly magnified. The researches of Mr. Goodsir have likewise thrown much light on the general process of secretion. He shows, by an admirably selected series of observations (chiefly on the lower animals), that secretion is a function of the nucleated cell.

If the membrane which lines the secreting portion of the internal surface of the ink-bag of Loligo sagittata (Lamarck) be carefully freed from adhering secretion by washing, it will be found to consist almost entirely of nucleated cells, of a dark brown or black colour. These cells are spherical or ovoidal. Their nuclei consist of cells grouped together in a mass. Between these composite nuclei and the walls of their containing cells is a fluid of a dark brown colour. This fluid resembles in every respect the secretion of the ink-bag itself. It renders each cell prominent and turgid, and is the cause of its dark colour.

The dilated terminal extremities of the ducts in the liver of Helix aspersa (Müller) contain a mass of cells. If one of these cells be isolated and examined, it presents a

nucleus consisting of one or more cells. Between the nucleus. and the wall of the containing cell is a fluid of an amber tint, and floating in this fluid are a few oil-globules. This fluid differs in no respect from the bile as found in the ducts of the gland. The liver of Modiola vulgaris (Fleming) contains masses of spherical cells. Between the nucleus and the wall of each of these cells a light-brown fluid is situated, bearing a close resemblance to the bile in the gastro-hepatic pouches. The nucleated cells which are arranged around the gastrohepatic pouches of Pecten opercularis are irregular in shape, and distended with a fluid resembling the bile. The hepatic organ which is situated in the loop of intestine of Pirena prunum (Fleming) consists of a mass of nucleated cells. These cells are collected in groups in the interior of larger cells or vesicles. These nucleated cells are filled with a light-brown bilious fluid. The hepatic organ situated in the midst of the reproductive apparatus, and in the loop of the intestine of Phallusia vulgaris (Forbes and Goodsir), consists of a number of vesicles, and each vesicle contains a dark-brown bilious fluid.

The hepatic cæca in the liver of Patella vulgata contains vesicles enclosing a body which consists of a number of nucleated cells, full of a dark fluid resembling the bile. The kidney of Helir aspersa (Müller) is principally composed of numerous transparent vesicles. In the centre of each vesicle is situated a cell full of a dead-white granular mass. This gland secretes pure uric acid. The ultimate elements of the human liver are nucleated cells. Between the nucleus and the cell-wall is a light-brown fluid with one or two oilglobules floating in it. The vesicular cæca in the testicle of Squalus cornubicus contain nucleated cells, which ultimately exhibit in their interior bundles of spermatozoa. The generative cæca of Echiurus vulgaris (Lamarck) contain cells full of minute spermatozoa. Aplysia punctata secretes from the edge and internal surface of its mantle a quantity of purple fluid. The secreting surface of the mantle consists of an arrangement of spherical nucleated cells. These cells are distended with a dark purple matter. The edge and internal surface of the mantle of the Janthina fragilis (Lamarck), the animal which supplied the Tyrian dye, secretes a deep bluish. purple fluid. The secreting surface consists of a layer of nucleated cells, distended with a dark purple matter. If an ultimate acinus of the mammary gland of the bitch be examined during lactation, it is seen to contain a mass of nucleated cells. These cells are generally ovoidal, and rather transparent. Between the nucleus and the cell-wall of each a quantity of fluid is contained, and in this fluid float one, two, thrce, or more oil-like globules, exactly resembling those of

the milk.

The secretion within a primitive cell is always situated between the nucleus and the cell-wall, and would appear to be a product of the nucleus.

The ultimate secreting structure then is the primitive cell, endowed with a peculiar organic agency, according to the secretion it is destined to produce. Mr. Goodsir names it the primary secreting cell. It consists, like other primitive cells, of three parts-the nucleus, the cell-wall, and the cavity. The nucleus is its generative organ, and may or may not, according to circumstances, become developed into young cells. The cavity is the receptacle in which the secretion is retained, till the quantity has reached its proper limits, and till the period has arrived for its discharge. Each primary secreting cell is endowed with its own peculiar property, according to the organ in which it is situated. In the liver it secretes bile; in the mamma, inilk, &c. The primary secreting cells of some glands have merely to separate from the nutritive medium a greater or less number of matters already existing in it. Other primary secreting cells are endowed with the more exalted property of elaborating from the nutritive medium matters which do not exist in it. The discovery of the secreting agency of the primitive cell does not remove the principal mystery in which this function has always been involved. One cell secretes bile, another milk; yet the one cell does not differ more in structure from the other, than the lining membrane of the duct of one gland from the lining membrane of the duct of another. The general fact however, that the primitive cell is the ultimate secreting structure, is of great value in physiological science, inasmuch as it connects secretion with growth, as phenomena regulated by the same laws. The force, of whatever kind it may be, which enables one primary formative cell to produce nerve and another muscle, by an arrangement within itself of the common materials of nutrition, is dentical with.

Instead of growth being a species of imbibing force, and secretion on the contrary a repulsive, the one centripetal, the other centrifugal, they are both centripetal. Even in their latter stages the two processes, growth and secretion, do not differ. The primary formative cell, after becoming distended with its peculiar nutritive matter, in some instances changes its form according to certain laws: and then, after a longer or shorter period, dissolves and disappears in the intercellular space in which it is situated; its materials passing into the circulating system if it be an internal cell, and being merely thrown off if it be an external cell. The primary secreting cell, again, after distension with its secretion, does not change its form so much as certain of the formative cells, but the subsequent stages are identical with those of the latter. It bursts or dissolves, and throws out its contents either into ducts or gland-cavities.

The general fact of every secretion being formed within cells, explains a difficulty which has hitherto puzzled physiologists, namely, why a secretion should only be poured out on the free surface of a gland-duct, or secreting membrane. We have attempted to illustrate Mr. Goodsir's views by the accompanying figure.

1

Fig. 5.

2

cupying the whole space of the cell (c); the remunder of the cell having often a striped or streaky appearance, and forming a lateral projection; this is seen in c, and in a more marked degree in d and e. In other fat-cells there were observed to be two vesicles, separated by a septum, against which they were partially flattened by pressure (g), & merely separated by a constriction in the external walls, an f. This form leads us to conclude that fat-cells increase by division. For the chemistry of this constituent we must refer to the article FAT, P. C., and to an early part of the present article. It is sufficient here to remark that the fat-vesicle of the human subject contains margarin, a solid, and oleis, a first fat. These sometimes separate spontaneously, presenting a very beautiful microscopic appearance. The margarin collects in a spot on the inner surface of the cell-membrane, and presents the appearance of a small star, whilst the olein occspies the remainder of the vesicle, unless when the quantity of fat in it is rather smaller than usual, in which case we may observe a little aqueous fluid between the olein and the cellmembrane. We have attempted to depict this separation in h.

2. Pigment. In certain parts of the animal organism we meet with definite and well-marked colorations, not depesdent on any peculiar arrangement of fibres, &c., but on the presence of pigment-granules of various colours. These granules are usually inclosed in cells, termed pigment-cells. In all races of men we find a most remarkable development of these cells on the inner surface of the choroid coat of the eye, where they form several layers known as the Pigmentom nigrum. They are probably always mingled with the epidermic cells, giving rise in the dark races to the deep colour of the skin; and presenting themselves in the white races in the form of freckles, the areola round the nipple, &c. The pag

2 Cells from the ink-bag of Loligo sagitta.

3. Cells from the liver of the Patella vulgata. In this instance the bile is contained in the cavities of the secondary cells, which constitute the nucleus of the primary cell.

4. Cells from the mamma of a bitch. In addition to their nuclei these cells contain milk-globules.

Persistent tissues. We now proceed to the histological and chemical investigation of the most important constituents of the human organism.

1. Adipose tissue is usually associated with areolar tissue (which see), the two being generally known collectively as cellular tissue. It must be distinguished from fat, the former being a membrane of extreme tenuity in the form of closed cells or vesicles, while the latter is the material contained within them. The membrane of the adipose vesicle does not exceed the 20,000th of an inch in thickness, and is quite transparent; it is moistened by watery fluid, for which it has a greater attraction than for the fat it contains. Each vesicle is a perfect little organ, varying, when fully developed, from the 300th to the 800th of a line; minute capillaries may be observed on their external surface. When fat-vesicles are deposited together in large numbers, as is usually the case, they assume a more or less regular polyhedric form from their mutual pressure.

Fig. 6.

polygonal form. The granules in their interior are extremely minute, retain their dark colour under high magnifying powers, but exhibit various forms. In the choroid membrane of the human eye their form is very regular; in the adult no nacier

Fig. 7.

Cells from the choroid coat of an adult.

cells from the foetus. The pigment-cells have not always can be seen, a structure which is obvious in corresponding a simple rounded or polygonal form; they sometima Fig. 8.

Similar cells from a foetus at the third month.

present remarkable stellate prolongations and other singular shapes, which we have attempted to depict in Fig. 9, repreFig. 9.

b

When the first traces of fat appear is not accurately known. In a well-formed five-months human foetus Valentin found in the subcutaneous cellular tissue of the sole of the foot not merely fat-cells, such as occur in adults, varying from the ordinary size to the 125th or 100th of a line, within and around which were numerous small vesicles (Fig. 6, a), but other forms which threw more light on their structure and development. In some the surrounding cell-membrane was much more distinct than as it occurs in adults (b). In

senting pigment-cells from a frog. a, b, c, d, e, and g, Fq. 9, are representations of various pigment-cells from its chored coat, while ƒ is intended to exhibit the stellar shape in which these cells occur on the skin of that animal. The nuclea

is sufficiently obvious in one of the cells in a, in c, d, e, and g..

Little is known of the chemistry of the animal pigments. Scherer has made three analyses of the black pigment from the eve of the ox, from which he concludes that it consists of

Carbon. Hydrogen. Nitrogen Oxygen

From these analyses it appears probable that the black pigment contains a larger amount of carbon than any other constituent of the animal body.

3. Horny tissues.-Under this general name are included not only true horns, but feathers, hairs, cuticle, the various forms of epithelium, and the crystalline lens. We shall confine our observations to the microscopic characters of epithelium and hair, and then briefly advert to the general chemical characters of the class.

The epithelium may be regarded as a delicate cuticle covering the free internal surfaces of the body, just as the epidermis (to which it is closely allied) invests the external surface. Some of the uses of the epithelial cells have been already noticed in our remarks on isolated cells, in addition to their obvious use in protecting the surfaces on which they are placed. This structure was first investigated by Henle (in Müller's Archiv,' 1838), and has been since carefully examined by Bowman (art. Mucous Membranes,' in Todd's Cyclopædia of Anatomy and Physiology,' 1842), Goodsir, and others. From the forms presented by the epithelial particles they have received different names. Henle divided them into pavement or tessellated epithelium, cylinder epithelium, and ciliated epithelium, and although they frequently run in one another, yet on the whole these distinctive terms are serviceable.

a

B

Fig. 10.

b

The three forms of epithelium. The pavement epithelium consists of broad flattened particles, or scales, having an angular outline and a nucleus; these scales form layers of extremely variable thickness. Fig. 10, A, shows very clearly how they are superimposed over one another, forming an effectual protection to the basement membrane beneath them. As a general rule the nucleus is large in proportion to the youth of the cell. In this figure we have attempted to exhibit these cells in two stages, a recent and a mature stage. In the young cells marked a the nucleus is relatively much larger than in b. This figure is intended to represent the epidermic scales of the frog; the larger cells, b, lying above the younger and smaller cells, a.

This form occurs on all synovial and serous membranes, and on most of the mucous membranes.

In the cylindrical epithelium the particles have the shape of small rods disposed endways on the basement membrane in a single layer. In consequence of their mutual compression they usually assume a prismatic rather than a cylindrical form, and hence Bowman applies the term prismatic to this form of epithelium. This form is perhaps best seem on the villi of the small intestine, or the conjunctival surface of the cornea of the eye. We have attempted to depict the latter in Fig. 10, B.

The ciliated epithelium is little more than cylinder epithelium on whose free surface numerous cilia or delicate filaments are observed in actual motion (Fig. 10, C). When in motion each filament appears to bend from its root to its point, returning again to its original state like corn moved by the wind. The motion of the cilia is not only quite independent of the will of the animal, but seems even to be indepentent of the life of the rest of the body; it has been seen after

| the death of the animal, and proceeding with perfect regularity in parts separated from the body. Dr. Carpenter states that ciliary movement has been observed fifteen days after death in the body of a tortoise. The motion may be readily observed in the oyster or muscle. In the human subject this form of epithelium exists in the air-passages with their various offsets, as the nasal cavities, eustachian tube, lachrymal ducts, &c., and in the upper part of the vagina, the uterus, and the fallopian tubes. Its purpose is evidently to propel fluids over the surfaces on which it occurs.

Hair. The shaft of the hair is that portion which is fully formed and projects beyond the surface. On examination we find it lodged in a follicular involution of the basement membrane (Fig. 11, a), which usually passes through the cutis into the subcutaneous areolar tissue. This hair-follicle is bulbous at its deepest part, like the hair which it contains. Its sides have a cuticular lining, b, continuous with the epidermis, and resembling the cuticle in the rounded form of its deep cells and the scaly character of the more superficial ones, which are here in contact with the outside of the hair, c. The hair grows from the bottom of the follicle, and the cells of the deepest stratum, there resting on the basement-membrane, are very similar to those which in other parts are transformed into scales of cuticle. A gradual enlargement occurs in these cells as they mount in the soft bulb of the hair, which indeed owes its size to this circumstance. If the hair is to be coloured, the pigment-grains are also here developed, for the most part in scattered cells, which may send out radiating processes; at other times, in a diffused manner around the nuclei of the cells generally. It frequently happens that the cells in the axis of the bulb become loaded with pigment at one period, and not at another; so that, as they pass upwards in the shaft, a dark central tract is produced of greater or less length, often only in irregular patches, and the hair appears here and there to be tubular, e. The shaft is much narrower than the bulb, and is produced by the rather abrupt condensation and elongation into hard fibres of the cells, both of those which contain pigment and those which do not. The granules of pigment assume a linear arrangement between the fibres, which are firmly united into a solid rod by a material similar, it may be supposed, to that which cements the scales of the cuticle.

The human hair has a proper bark, or cortex, formed in the following way :-A single layer of the cells immediately surrounding those about to form the fibrous tissue of the shaft are seen near the bottom of the follicle to assume an imbricated arrangement (Fig. 11, c), and gradually to mount on the hair, becoming more compressed against it in their ascent, until they form upon its surface a thin transparent colourless film, in which the overlapping of the delicate cells is still exhibited by elegant and exceedingly fine sinuous Fig. 11.

Bulb of a small black hair from the scrotum, seen in section. a, basement-membrane of the follicle; b, layer of epidermic cells resting upon it and becoming more scaly as they approach c, a layer of imbricated cells forming the outer lamina or cortez of the hair. These imbricated cells are seen more flattened and compressed, the higher they are traced on the bulb Within the cortex is the proper substance of the hair, consisting at the base, where it rests on the basement membrane, of small angu ar cells, scarcely larger than their nuclei. At d these cells are more bulky and the bulb consequently dantly. Above d they assume a decidedly fibrous character and become conthicker: there is also pigment developed in many of them more or less abun densed. e, a mass of cells in the axis of the hair, much loaded with pigment.

cross lines (Fig. 12, d, d'). The fibrous interior and this peculiar cortex together compose the shaft of the hair. By the continual emergence of fresh portions of the shaft from the follicle, fragments of the cuticular lining of the latter are apt to be drawn up upon the hair, aided probably in this by the imbrication of its surface, and are often found clinging around it for some way; but they are not to be regarded as any part of the hair itself. From the preceding description it will be evident that the fibrous part of the hair is a peculiar development of the cuticular cells resting on the bottom of the follicle, that the imbricated cortex is formed by a single series differently developed at the circumference of these, and that beyond this series comes the cuticular lining of the follicle, so that the hair is neither covered nor underlaid by cuticle, but it is in fact the modified cuticle of the bottom of the follicle.

Fig. 12.

away the areola: tissue with which it is associated, it seems, when examined under the microscope, to consist of extremely delicate fibrillæ running parallel to one another, and taking an undulating course. There is however reason to believe it does not in reality consist of a bundle of fibrillæ, but that it in simply a mass with longitudinal parallel streaks, and whack has a tendency to split up in a longitudinal direction (Fig. 14, a). Yellow fibrous tissue differs in many essential points from the preceding form. It is remarkably elastic, is of a yellow colour, and is arranged in bundles or fibres, invested by thin sheath of areolar tissue. In man we find it extended between the lamina of the vertebræ, in several other ligaments, and in the transversalis fascia of the abdomen. It forms the ligamentum nuchae of animals. Examined under the microscope it is seen to consist of fibres varying in da meter from the 5000th to the 10,000th of an inch. They b furcate or even divide into three, and freely anastomose with each other.

Fig. 13.

a, Transverse section of a hair of the head, showing the exterior cortex, the fibrous tissue with its scattered pigment, and a central space filled with pigment. b, a similar section of a hair at a point where no aggregation of pigment in the axis exists. c, longitudinal section, without a central cavity, showing the imbrication of the cortex, and the arrangement of the pigment in the fibrous part. d, surface showing the sinuous transverse lines formed by the edges of the cortical scales. d', a portion of the margin, showing their im

brication.

Yellow fibrous tissue showing the curly and branched disposition of its Strilin Areolar tissue is dispersed over almost every portion of the body, being the substance most commonly (but incorrectly) termed cellular tissue. The following are the microscope characters of this tissue, as described by Bowman and Todd:When a fragment is examined, it presents an inextricable interlacement of tortuous and wavy threads, intersecting one another in every possible direction. They are of two kinds. The first are chiefly in the form of bands of very unequal thickness, and inelastic. Numerous streaks are visible in them, not usually parallel with the border, though taking a general longitudinal direction. These streaks, like the bands themselves, have a wavy appearance, but can be rendered straight by being stretched. The streaks seen have more the marks of longitudinal creasing than a true separation into threads; for it is impossible to tear up the band into filaments of determinate size, although it manifests a decided tendency to tear lengthways. The larger of these bands are often as wide as the 500th of an inch; the smaller can only be detected with high powers. These are the white fibrous element. The others are long, single, elastic, branched filaments, with a dark, decided border, and disposed to curl when not put on the stretch. They interlace with the others, but appear to have no continuity of substance with them. They are most commonly about the 8000th of an inch in diameter. These form the yellow fibr us element.

From the analyses of Van Laer it appears that the average amount of sulphur in human hair is 5 per cent. From a series of well-devised experiments he concludes that 'the hair consists essentially of

1. A connecting medium consisting of a tissue yielding gelatin, and represented by the formula C1s Ho N3 Os ;and

2. Of bisulphuret of protein, Co H31 NS O12 Se

'The large amount of sulphur is the cause of its colour being affected by various metallic salts. As there is no constant difference to be observed in the results obtained by the analysis of hair of various tints, it is to be presumed that the colour is dependent on peculiar arrangements of the ultimate particles.' Hair further contains about 0.4 per cent. of peroxide of iron, which is supposed by Van Laer to be chemically combined with the protein.

4. Fibrous and Areolar tissues.-Fibrous tissue is now usually considered under two heads, namely as the white and yellow tissue.

The fibrous tissue occurs in ligaments, tendons, and es requiring great strength. On carefully dissecting