This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1885). The sternal apparatus is not less interesting to the osteologist. The anterior extremity of the sternum is osseous, and considerably prolonged forwards, to be articulated with the clavicles, and thus afford a support to the anterior extremity. Behind this it becomes cartilaginous, and affords attachment to the sternal ribs, which enter into the composition of the thorax: it does not, however, terminate at the posterior margin of the thoracic cavity, but is continued along the mesial line of the abdomen quite to the pubis, and gives off eight abdominal sternal ribs, to which no dorsal correspondents are met with. These abdominal ribs serve to support the muscles of the abdomen, and here present their maximum of development: rudiments of them are, however, still met with in the higher animals; and even in the human subject we find, in the transverse tendinous bands which intersect the substance of the rectus muscle of the abdomen, the last remains of these appendages to the sternal portion of the skeleton.
(1886). In the anterior extremity of the Crocodile we have most of the parts enumerated as entering into the composition of a perfect or typical skeleton: the shoulder, however, is composed of only two pieces, the scapula and the clavicle, the last of which articulates with the sternum; the bones of the arm, fore-arm, and hand are completely developed.
(1887). The posterior extremities are fully formed, the pelvis being connected by means of the ossa ilii to the transverse processes of two vertebrae, which therefore, as we have seen, constitute the sacrum.
(1888). In examining the bones which enter into the composition of the head of the Crocodile, or indeed of most Reptiles, the anatomist finds his studies much facilitated by the circumstance that the sutures separating the individual bones never become obliterated, so that the elements of this portion of their skeleton remain permanently detached and separate; and for this reason we shall take the present opportunity of going a little into detail concerning the composition of the skull of the Crocodile, as it is well calculated to illustrate the real structure of the cranium in the Vertebrata generally.
(1889). The bones belonging to the face are easily recognized: the Intermaxillary (17), the maxillary (18), and the nasal (20), the zygo-matic (b), and the lacrymal (c), all occupy their usual relative positions. The roof of the mouth is formed, as in Man, anteriorly by an elongated process of the upper jaw (18), and posteriorly by the palate-bone (22.) (1890.) The frontal consists of five pieces, viz. the principal frontal (l), which probably, in the foetus, consisted of two lateral halves, the anterior frontal (2, 2) and the posterior frontal (4, 4).
Fig. 332. Skull of the Crocodile.
(1891). The parietal (7) is, as is generally the case in Reptiles, represented by a single bone.
(1892). The occipital consists of four pieces, which remain permanently detached; namely, the basilar (5), the two lateral occipital (10), and the superior occipital, placed above the foramen magnum.
(1893). The sphenoid, which in Man is regarded as a single bone, is here represented by several distinct parts. The body is divided into two portions (6), called respectively the anterior and the posterior sphenoids. The great or temporal aide (11) are also separate bones, as also are the internal pterygoids (25).
(1894). A bone (24), which is not met with either in Mammalia or Birds, passes from the internal pterygoid to the point of junction between the zygomatic, the maxillary, and the posterior frontal: this has been named by Cuvier the transverse bone.
(1895). The ethmoid and the vomer (16) are but very imperfectly ossified, so that the septum between the nostrils is in the skeleton extremely incomplete, and the sense of smell of course proportionately obtuse.
(1896). But the most interesting of the cranial bones is the temporal, which, although considered as one bone by the human osteologist, is in Reptiles evidently composed of at least four distinct and separate parts. These are, 1st, the petrous bone (fig. 332, a, e), which partially encloses the organ of hearing; 2ndly, the tympanic bone (a), which supports the membrana tympani; 3rdly, the mastoid bone (12), which is the homo-logue of the mastoid process of Man; and 4thly, the temporal bone, properly so called (23), which represents the squamous portion of the human temporal bone.
(1897). Each lateral division of the inferior maxilla of Reptiles is separable into at least five, and generally six pieces, which are united together by suture; these are named the dental (34), which support the teeth, the angular (36), the opercular (37), the articular (35), and two small pieces seen upon the inner surface of the jaw.
(1898). Having thus described at some length the composition of the skeleton in the Crocodile, which we have chosen for minute analysis, as being the type of the Saurian Reptiles, we shall now proceed to examine the osteology of the other orders, so as to appreciate more correctly the peculiarities of structure that they individually exhibit.
(1899). In the Amphibia, as for example in the Frog, one of the most striking circumstances connected with their history is the extraordinary change which takes place in the condition of every part of the framework of the body during the evolution of the tadpole and its metamorphosis into the perfect frog.
(1900). The skeleton of a Tadpole is, in every particular, that of a fish: its texture is soft and cartilaginous, the caudal portion of the spine prolonged and flexible; neither are there any external limbs connected with the vertebral column, so as to trammel the lateral movements of the tail; and yet in the mature Frog (fig. 333) let the reader observe the amazing difference. The head, it is true, still preserves somewhat of the character of that of the fish, especially in the disproportionate development of the face when compared with the size of the cranial cavity; but all the bones of the spine have become consolidated into ten vertebrae, firmly connected together by strong articulations, while the flexible tail of the tadpole has become converted into a strong and immoveable os coccygis, composed of a single piece.