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.
(1875). The transition from the Ophidia to the Lizards (Sauria), composing the third order of Reptiles, is very gradually accomplished by several intermediate forms, in which the first buddings of legs make their appearance; and these locomotive organs, becoming more and more completely developed in other genera, at length conduct us from the flexible and apodous Serpents to the strong and four-footed Reptiles which are the types of the Saurian division. The progressive development of the locomotive extremities is not a little curious: even among some of the Serpents properly so called, as, for example, in the Anguis fragilis of our own country, the rudiments of these limbs may be detected beneath the skin; more especially those of the hinder extremity, wherein a little pelvis and femur may be distinctly recognized, while a minute sternum, clavicle, and scapula indicate the first appearance of the thoracic legs.
(1876). In Bimanes, the lowest of the Saurian genera, two little feet, each provided with four toes, are appended to the framework of the shoulder; and in Seps, which equally possesses the body of a serpent, all four extremities first make their appearance externally. As the legs become increased in their relative size and importance, the trunk is proportionately shortened and its flexibility diminished, until at length we are conducted, almost by imperceptible gradations, to the strong and voracious Crocodiles, the most perfect of the Reptile families.
(1877). The fourth order of Reptiles (Cheloistia) comprises a series of animals of most anomalous conformation, in which the greater part of the skeleton is brought quite to the exterior of the body, and the limbs are absolutely enclosed within the cavity formed by the ribs. Such are the Tortoises and the Turtles; but as we shall describe the anatomy of these animals more at length hereafter, we need only in this place point out to the reader their outward form and general appearance.
(1878). Commencing our researches concerning the internal organization of this extensive class by examining the osteology of the Reptilia, we shall, as we have hitherto done, select one skeleton for special ex- * amination; and afterwards, taking that as a standard of comparison, observe the most conspicuous modifications of structure met with in the different divisions of this important group.
(1879). The skeleton we choose for particular description is that of the Crocodile, one of the most interesting that can possibly be offered to the contemplation of the comparative anatomist, inasmuch as it exhibits, developed to a medium extent, a greater number of the elements which we have supposed to enter into the composition of a perfect or typical skeleton than any other with which we are acquainted: we therefore beg the attention of the student while we investigate this important piece of osteology.
(1880). A glance at the skeleton of the Crocodile (fig. 331) at once shows us that, in consequence of the addition of a thorax, and the connexion which now necessarily exists between the pelvis and the spine, the vertebral column becomes divisible into distinct regions: viz. the cervical, containing seven vertebra?; the dorsal, formed by those vertebrae which support the thoracic ribs; and the lumbar vertebrae intervening between these and the sacrum. The number of bones entering into the composition of the sacrum, that is, which are connected with the ossa ilii of the pelvis, are in this case two in number; while, behind these, six-and-thirty vertebrae enter into the composition of the tail.
(1881). In the cervical, dorsal, lumbar, and sacral regions, no inferior spinous processes exist; but in the caudal portion of the vertebral column these elements are found greatly developed, as in fishes, and obviously with the same intention, namely to increase as much as possible the vertical extent of the tail, and thus convert this part of the body, which is here of extraordinary length and great flexibility, into a powerful instrument of propulsion.
(1882). The transverse processes of the cervical vertebrae are remarkably large, and so extended that they materially interfere with the lateral movements of the neck, - an arrangement evidently designed to afford a sufficient extent of insertion for the powerful muscles of the cervical region.
(1883). The thorax is composed of a sternum and two sets of ribs, - one set being articulated with the transverse processes of the dorsal vertebrae, and hence called dorsal ribs, while the others, being fixed to the sides of the sternum, are named sternal ribs: the contiguous extremities of the dorsal and sternal ribs are, moreover, united by intervening cartilages, which, as they are generally more or less perfectly ossified in the adult Crocodile, might almost be regarded as additional elements of the thorax.
Fig. 331. Skeleton of the Crocodile.
(1884). The posterior dorsal ribs are far less perfectly developed than those situated more anteriorly; and it is not a little interesting to observe how gradually, even in the same skeleton, the transition is effected from the simple condition already noticed in the ribs of fishes, in which each rib is merely appended to the extremity of the transverse process of a vertebra, to ribs perfectly adapted to enter into the composition of a true thoracic cavity, and united by a double articulation both with the transverse processes and the bodies of the vertebras. The head of the last rib of the Crocodile is, in fact, simple, and merely articulated with the apex of the transverse process of the corresponding vertebra; the next is slightly bifid at its origin, but both the divisions are still connected with the transverse process; as we advance still further forwards, the division of the origin of the rib becomes more and more decided, until at length, at about the fifth rib, we have two distinct heads, one firmly articulated with the body of the vertebra, the other with the transverse process - presenting an arrangement precisely similar to that met with in the structure of the thorax of a bird.