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anchylosed with the substance of the jaw. The tongue is fleshy, well-developed, with the apex more or less free. The posterior nares are protected by a soft palate, and the larynx by an epiglottis: the rings of the trachea are generally cartilaginous and incomplete behind: there is no inferior larynx. The oesophagus is continued without partial dilatations to the stomach, which varies in its structure according to the nature of the food, or the quantity of nutriment to be extracted therefrom.
The trunk-vertebrae of Mammalia have their bodies ossified from three centres, and present for a longer or shorter period of life a discoid epiphysis at each extremity. They are articulated by concentric ligaments with interposed glairy fluid forming what are called the intervertebral substances; the articulating surfaces are generally flattened, but sometimes, as in the necks of certain Ruminants, they are concave behind and convex in front. The cervical vertebra are seven in number, neither more nor less. The lumbar vertebrae are more constant and usually more numerous than in other classes of vertebrate animals. The atlas is articulated by concave articular processes to two convex condyles, which are developed from the ex-occipital elements, or neurapophyses, of the last cranial vertebra. The tympanic element of the temporal bone is restricted in function to the service of the organ of hearing, and never enters into the articulation of the lower jaw. The olfactory nerves escape from the cranial cavity through numerous foramina of a cribriform plate. The optic foramina are always distinct from one another.
The scapula is generally an expanded plate of bone; the coracoid, with two (monotrematous) exceptions, appears as a small process of the scapula. The sternum consists of a narrow and usually simple series of bones: the sternal portions of the ribs are generally cartilaginous and fixed to the vertebral portions without the interposition of a distinct articulation: there are no gristly or bony abdominal ribs or abdominal sternum. The pubic and ischial arches are generally complete, and united together by bony confluence on the sternal aspect, so that the interspace of the two pelvic arches is converted into two holes, called 'foramina obturatoria.'
The sclerotic coat of the eye is a fibrous membrane, and never contains bony plates. In the quantity of aqueous humour and the convexity of the lens Mammals are generally intermediate between Birds and Fishes. The organ of hearing is characterized by the full development of the cochlea with a lamina spiralis: there are three distinct ossicles in the tympanum; the membrana tympani is generally concave externally; the meatus auditorius externus often commences with a complicated external ear, having a distinct cartilaginous basis. The external apertures of the organ of smell are provided with moveable cartilages and muscles, and the extent of the internal organ is increased by accessory cavities or sinuses which communicate with the passages including the turbinated bones.
There are few characters of the osseous system common, and at the same time peculiar, to the class Mammalia. The following may be cited:—
1. Each half or ramus of the mandible consists of one bony piece developed from a single centre: the condyle is convex or flat, never concave. This has proved a valuable character in the determination of fossils.
2. The second or distal bone, called 'squamosal,' in the 'zygomatic' bar continued backward from the maxillary arch, is not only expanded, but is applied to the side-wall of the cranium, and developes the articular surface for the mandible, which surface is either concave or flat.
3. The presphenoid is developed from a centre distinct from that of the basisphenoid.
In no other class of vertebrate animals are these osteological characters present.
The cancellous texture of mammalian bone is of a finer and more delicate structure than in Keptiles, and forms a closer network than in Birds. The microscopic radiating cells are relatively smaller and approach more nearly to the spheroid form.
The Mammalia, like Reptilia and Pisces, include a few genera and species that are devoid of teeth; the true anteaters (Myrmecophaga), the scaly anteaters or pangolins (Manis), and the spiny monotrematous anteater (Echidna), are examples of strictly edentulous Mammals. The Ornithorhynchus ha3 homy teeth, and the whales (Balcena and Balamoptera) have transitory embryonic calcified teeth, succeeded by whalebone substitutes in the upper jaw. The female Narwhal seems to be edentulous, but has the germs of two tusks in the substance of the upper jaw-bones; one of these becomes developed into a large and conspicuous weapon in the male Narwhal, whence the name of its genus Monodon.
The examples of excessive number of teeth are presented, in the order Bruta, by the priodont Armadillo, which has ninety-eight teeth: and in the Cetaceous order by the Cachalot, which has upwards of sixty teeth, though most of them are confined to the lower jaw; by the common Porpoise, which lias between eighty and ninety teeth: by the Gangetic Dolphin, which has one hundred and twenty teeth; and by the true Dolphins (Delphinus), which have from one hundred to one hundred and ninety teeth, yielding the maximum number in the class Mammalia.
When the teeth are in excessive number, as in the Armadillos and Dolphins above cited, they are small, equal, or sub-equal, and usually of a simple conical form.
In most other mammals particular teeth have special forms for special uses; thus, the front teeth, (figs. 2 and 3, »,) from being commonly adapted to effect the first coarse division of the food, have been called cutters or incisors; and the back teeth, (ib. m,) which complete its comminution, grinders or molars; large conical pointed teeth situated behind the incisors, and adapted, by being nearer the insertion of the biting muscles, to act with greater force, are called holders, tearers, laniaries, or more commonly canines, (ib. c,) from being well developed in the Dog and other Carnivora.
It is peculiar to the class Mammalia to have teeth implanted in sockets by two or more fangs; but this can only happen to teeth of limited growth, and generally characterizes the molars and premolars: perpetually growing teeth require the base to be kept simple and widely excavated for the persistent pulp. In no mammiferous animal does anchylosis of the tooth with the jaw constitute a normal mode of attachment. Each tooth has its peculiar socket, to which it firmly adheres by the close co-adaptation of their opposed surfaces, and by the firm adhesion of the alveolar periosteum to the organized cement which invests the fang or fangs of the tooth.
True teeth implanted in sockets are confined, in the Mammalian class, to the maxillary, premaxillary, and mandibular or lower maxillary bones, and form a single row in each. They may project only from the premaxillary bones, as in the Narwhal; or only from the lower maxillary bone, as in Ziphius; or be limited to the superior and inferior maxillaries and not present in the premaxillaries, as in the true Ruminantia and most Bruta (Sloths, Armadillos, Orycteropes). In most Mammals teeth are situated in all the bones above mentioned.
The teeth of the Mammalia usually consist of hard unvascular dentine, defended at the crown by an investment of enamel, and everywhere surrounded by a coat of cement.
The coronal cement is of extreme tenuity in Man, Quadrumana and the terrestrial Carnivora; it is thicker in the Herbivora, especially in the complex grinders of the Elephant.
Vertical folds of enamel and cement penetrate the crown of the tooth in the ruminating and many other Ungulata, and in most Rodents, characterizing by their various forms the genera of those orders.
No Mammal has more than two sets of teeth. In some species the tooth-matrix does not develope the germ of a second tooth, destined to succeed the one into which the matrix has been converted; such a tooth, therefore, when completed and worn down, is not replaced. The Sperm Whales, Dolphins, and Porpoises are limited to this simple provision of teeth. In the Armadillos and Sloths, the want of generative power, as it may be called, in the matrix is compensated by the persistence of the matrix, and by the uninterrupted growth of the teeth.
In most other Mammalia, the matrix of the first-developed tooth gives origin to the germ of a second tooth, which sometimes displaces the first, sometimes takes its place by the side of the tooth from which it has originated.
All those teeth which are displaced by their progeny are called 'temporary,' deciduous, or milk-teeth, (figs. 2 and 3, d, 1...4); the mode and direction in which they are displaced and succeeded, viz. from above downwards in the upper, from below upwards in the lower, jaw, in both jaws vertically—are the same as in the Crocodile; but the process is never repeated more than once in any mammalian animal. A considerable proportion of the dental series is thus changed; the second or 'permanent' teeth having a size and form as suitable to the jaws of the adult, as the 'temporary' teeth were adapted to those of the young animal.
Those permanent teeth, which assume places not previously occupied by deciduous ones, are always the most posterior in their position, and generally the most complex in their form. The term 'molar' or 'true molar' is restricted to these teeth (fig. 2 and 3, m). The teeth between them and the canines are called 'premolars,' (ib. p); they push out the milk-teeth, (ib. d,) and are usually of smaller size and simpler form than the true molars.
Thus the class Mammalia, in regard to the times of formation and the succession of the teeth, may be divided into two groups, viz. Monophyodonts1 or those that generate a
1 n6vos, once; 0tfti>, I generate; 6dois, tooth.