(1901). No ribs whatever are met with in the Frog; and even in those Amphibia which are possessed of these elements of the skeleton, they are mere rudiments appended to the extremities of the transverse processes of the vertebras. The sternum, however, is largely developed, and gives extensive attachment to the muscles of the abdomen. The anterior extremities are supported by a semicartilaginous zone, in which the three elements of the shoulder (the scapula, the clavicle, and the coracoid bone) are distinctly recognizable; and the bones of the arm, fore-arm, and hand are very perfectly formed.

(1902). The pelvis is large and firmly ossified, in correspondence with the strength and magnitude of the hinder extremity, the ossa ilii being articulated to the ends of the transverse processes of the last vertebra, which from this circumstance maybe called the sacrum. The tibia and fibula are consolidated into one bone; while two of the bones of the tarsus (the astragalus and the os calcis) are so excessively elongated that they might almost be taken for a second tibia and fibula, did not their position indicate their real nature.

(1903). One circumstance is remarkable in the construction of the shoulder-joint of these reptiles, which are found to have a strong ligament passing between the head of the humerus and the scapula, exactly in the same manner as the ligamentum teres of the human hip-joint. The use of such a deviation from the ordinary structure of the articulation is obvious: the Frog, as it alights from those long and vigorous leaps which form its ordinary mode of progression, receives the whole shock of its fall upon its fore-legs, and thus this ligament becomes needful as an additional security to the articulation in question.

(1904). The skeleton of an Ophidian Reptile presents a strange contrast to that of the Batrachian, last described. Taking the Boa Constrictor as an example of this order, we find the spine of this enormous serpent composed of three hundred and four distinct vertebra), of which two hundred and fifty-two support ribs: flexibility is therefore abundantly provided for in the construction of these lithe and elegant beings, inasmuch as the division of their spinal column into so many pieces allows the utmost pliancy in any required direction. Flexibility, however, is not the only condition requisite in this case: strength and precision of movement are equally indispensable; and the question is, how are these apparently opposite qualities to be so combined and associated as not in the slightest degree to interfere with each other? The mechanism conspicuous in the construction of the spine of a Serpent is in this respect truly admirable. The anterior extremity of the body of every vertebra is rounded into a smooth and polished ball (fig. 334, c), which exactly fits into a hemispherical cup excavated in the substance of the vertebra next succeeding: a perfect ball-and-socket joint is thus formed between every vertebra and that which precedes or follows it; and thus the spine is rendered capable of the utmost latitude of movement, and offers at the same time a firm purchase to the muscles acting upon the vertebral column. To provide, however, against undue extent of motion in certain directions, we now meet with other processes derived from the vertebral arches: in addition to those given merely as levers for the attachment of muscles, secondary apophyses, called oblique or articulating processes, become developed; and contiguous vertebrae being likewise moveably connected together by means of these appendages, unnecessary flexure is not allowed, and all danger of dislocation prevented.

Skeleton of the Frog.

Fig. 333. Skeleton of the Frog.

Vertebrae and Ribs of Boa.

Fig. 334. Vertebrae and Ribs of Boa.

(1905). Serpents, being entirely deprived of external limbs, have neither shoulder nor pelvis, their ribs alone affording them the means of progression. These extend on each side in an uninterrupted series from the,first vertebra behind the head to the origin of the tail, so that the division of the spine into regions is here out of the question. Every rib is attached at its origin by a kind of ball-and-socket joint (fig. 334, a, b) to the extremity of the corresponding transverse process of a vertebra, and is therefore freely moveable. There is no sternum here, neither are there sternal ribs; but the dorsal ribs, wielded as they are by innumerable and powerful muscles connected with them, literally perform the office of internal legs, and materially assist the creature in progression.

(1906). Having already enumerated the bones which enter into the composition of the cranium of a Saurian Reptile, it would be superfluous again to mention in detail those met with in the skull of a Serpent, more especially as they will be easily recognized by a glance at the annexed figure, in which the corresponding bones are all indicated by the same references. One peculiarity only requires special notice, namely the extreme mobility of the principal bones of the face, and more particularly of the pieces composing the lower jaw, by which provision these reptiles are enabled to swallow entire animals of astonishingly large dimensions when compared with the size of their mouth.

(1907). In order to allow of this, the bones composing the superior maxilla (fig. 335,17,18) are only loosely joined together by ligamentous bands; and even the arches of the palate are moveable. The two halves of the lower jaw (34, 34) are connected together at the symphysis by a ligament so loose and elastic that separation to a great extent is easily allowed; and, moreover, those two elements of the temporal, the mastoid (12) and the tympanic (a), which form the bond of connexion between the inferior maxilla and the cranium are here lengthened out into long pedicles, so that by their mobility the entrance to the throat can be dilated in a surprising manner, and prey of apparently very disproportionate bulk thus introduced into the stomach.