(2151). The thorax is enclosed by ribs, which in structure, and in their mode of connexion with the dorsal vertebras, resemble those of Man. At its dorsal extremity each rib is articulated by its head to the bodies of the vertebras, and to the intervertebral substance; while its tubercle, or the representative of the second head of the rib of a Bird, is move-ably connected with the corresponding vertebral transverse process. There are no sternal ribs; but these are represented by cartilaginous pieces, whereby, towards the anterior part of the thorax, each rib is attached to the side of the sternum: posteriorly, however, this connexion does not exist. The anterior ribs are therefore called true ribs, and the posterior, false, or floating ribs, precisely as in the human skeleton.

(2152). The sternum is composed of several narrow pieces, placed in a line behind each other along the middle of the breast. These pieces are generally consolidated: by their lateral margins they give attachment anteriorly to the clavicles, if these bones be present, and, behind these, to the costal cartilages of the true ribs.

(2153). From the whole arrangement of the thorax, it is evident that the ribs are capable of extensive movements of elevation and depression, whereby the capacity of the whole thoracic cavity may be increased or diminished - movements which, aided by those of the diaphragm, draw in and expel the air used for respiration.

(2154). The anterior extremity is appended to a broad scapula, generally unconnected with the rest of the skeleton, except by muscular attachments. In quadrupeds that use this extremity as an instrument of prehension or of flight, a clavicle is interposed between the scapula and the sternum; but most frequently this element of the shoulder is deficient, and even the coracoid bone, if a vestige of it remains at all, is reduced to a mere appendage to the scapula, known to the human anatomist as the coracoid process. The rest of the limb presents the arm, the fore-arm, the carpus, metacarpus, and phalanges; but these are so altered in appearance in different orders, that no general description will suffice, and we must therefore defer this part of our inquiry for the present.

(2155). In the posterior extremity there is equal dissimilarity in the construction of the distal portions of the limb; but the pelvis, although much modified in form, consists of the same pieces as in the human subject, and in like manner has the pubic arch and foramina fully completed.

(2156). The cranium and face are made up of numerous bones, easily recognizable, as they correspond in their general arrangement with those composing this part of the skeleton in the lower Vertebrata. Their development in the facial region is large, in proportion to the strength of the muscles moving the lower jaw; and they are so disposed as to form buttresses to resist the powerful pressure of the teeth, as well as to enclose cavities wherein are lodged the organs connected with the senses of smell and of vision. One example will answer our present purpose, and we have selected the skull of the Pig as one calculated to show a medium development of the whole series.

(2157). In the face we find on each side two bones entering into the composition of the upper jaw, into which teeth are implanted; these are the superior maccillary (fig. 380,18), and the intermaxillary (17.) These bones, moreover, bound extensively the cavity of the nose, and, together with the palatine process of the palate bone (fig. 381, 22), constitute the bony palate, or roof of the mouth. The nasal bones (fig. 380,20, 20) complete the upper part of the face; and, being in contact along the mesial line, arch over the nasal chamber.

Skull of the Pig.

Fig. 380. Skull of the Pig.

(2158). The orbit is bounded anteriorly by the lacrymal bone (c), and the jugal or malar bone (6.) Its posterior boundary is generally wanting, as the external angular processes of the jugal and frontal bones do not meet.

(2159). The orbital cavity is principally formed by processes derived from the os frontis, the sphenoid, the lacrymal, and the malar bone; the ethmoid and the palatine rarely entering into its composition.

(2160). The os eihmoides, the vomer, and the turbinated bones will be described minutely when we speak of the olfactory apparatus, which they contribute to form.

(2161). The inferior maxilla in Mammals is characterized by two circumstances, which distinguish it from that of other Vertebrata. It consists, in the first place, of only two lateral pieces, exactly similar to each other, joined together at the chin by a symphysis in many orders; but in others even this symphysis is obliterated at an early age, and in the adult the two lateral halves would seem to form but one piece.

(2162). Another character peculiar to the lower jaw of a Mammal is, that it is moveably articulated with the temporal bone by means of a convex and undivided condyle.

(2163). These marks, identifying the mammiferous lower jaw, ought to be well remembered by the palaeontologist.

(2164). "We shall hereafter have occasion to describe the teeth that arm the jaws of the different tribes of quadrupeds; and therefore we now proceed to examine their cranial cavity, and the bones that enter into its formation.

(2165). The frontal bones (figs. 380, 381, 1, 1) are generally two in number; and even when, as in Man, they seem to form but one bone, the two lateral halves are produced from separate points of ossification, and only coalesce as age advances: sometimes, indeed, even in the adult, they remain permanently separated by suture.

Section of the skull of the Pig.

Fig. 381. Section of the skull of the Pig.

(2166). The parietal bones (figs. 380, 381, 7, 7) occupy their usual position; and although generally double, as in the human skeleton, they are not unfrequently consolidated together, even at an early age, so as to represent but a single bone.