Page:The American Cyclopædia (1879) Volume XIII.djvu/440

 426 PHILOSOPHICAL ANATOMY and have taxed the ingenuity of most com- parative anatomists, and tried to the utmost the patience of their readers ; in his " Com- parative Anatomy of Fishes" and "Homolo- gies of the Vertebrate Skeleton," Owen labors very hard, and not always very satisfactorily, to reduce everything to his archetype. It will be interesting and instructive to give a few of the most striking characters of the skeletons of the different classes of vertebrates, accord- ing to Owen. In the fish not only the jaws, but the arms and legs, may belong to the skull, which accordingly is developed out of propor- tion to the rest of the body ; the diverging ap- pendages of the frontal vertebra are the chain of opercular bones, and of the parietal the branchiostegal rays ; of the first spinal vertebra the pleurapophysis is short and simple, and the haemapophysis is the clavicle ; in the abdominal segments the pleurapophyses support simple rays as diverging appendages, and the haemal arches are fibrous ; the haemapophysis of the pelvic segment is ossified into an ischium sus- taining the ventral fins or posterior limbs, in some instances united to a rib ; this pelvic arch is most remarkably changed in position, being as above mentioned in the so-called abdominal fishes (like the salmon, herring, and pike), or joined to the scapular arch as in the thoracic fishes (like the cod and perch families), accord- ing as the ischium is joined to the coracoid by a longer or shorter development. The bony and fibrous parts of the haemal arches contract rapidly beyond the abdomen ; the parapophyses increase gradually, curve downward, and com- plete the arch as in the cod, or the pleurapo- physes contribute to form it with them as in lepidosteus, or the arch is closed by the former, with the latter anchylosed below and diverging at the points, as in the tunny. The bodies of some of the terminal segments in typical osseous fishes are consolidated together, and support several neural and haemal arches and spines, which form the more or less expanded base of the caudal fin. The dorsal, anal, and caudal fins are folds of the skin supported on spines between the neural and haemal spines to which the fin rays are articulated ; they form no part of the typical vertebrate skeleton, and are peculiar to fishes. As compared to his ar- chetype figure, the fish skeleton departs from it in the excess of development, principally in the diverging appendages of the cranium, and in the arrest of development in most of the other segments ; the principle of repetition predomi- nates, and the segments resemble each other more than in the higher classes. In the reptile skeleton, the haemal arches of the anterior two cranial vertebras, the jaws, are more developed, while that of the parietal is feebly so, and they are more or less displaced backward; in the occipital segment the haemal or scapular arch is still further displaced backward and entire- ly separated; to it is attached an additional single bone, the humerus, and the divisions of the terminal segments are reduced to five, a number not surpassed in any of the higher vertebrates. A part of the body of the atlas is developed separately, and is united to the second cervical vertebra, forming the odontoid process ; the nine segments after the cranium are cervical vertebrae, movably articulated, the haemal arches not being ossified, and the pleu- rapophyses feebly developed, but free or float- ing ; the nine to twelve following are dorsal vertebrae, the elongated ribs with the haemal arch completing the circle, the pieces of which are movable ; the next three are the lumbar, without free and bony ribs, but with haemal arches ; the next two, united, form the sacrum, bearing the pelvic arch, consisting of pleura- pophyses (ilium), haemapophyses (ischium and pubis), with the divergent appendages of the posterior limbs, a higher development than in fishes ; beyond the sacrum all the vertebraa are caudal, in which the pleurapophyses be- come gradually shorter, a few of the first at- tached to diapophyses, and the haemapophysea articulated between and to two vertebral bod- ies. In this class we see for the first time regions of the body. In the bird skeleton the premaxillary is much more developed than the maxillary, the reverse of what is seen in rep- tiles ; the greater volume of the brain requires an increased cranial cavity, which is obtained by the expansion of the neural arches and spines without the addition of any new bones ; the cervical segments have short and free pleurapo- physes or ribs, which are early united to the neural arches, forming numerous simple ver- tebrae, and giving length and flexibility to the neck ; the detached haemapophyses of the atlas are usually joined at their extremities, forming a thin osseous arch, the f urcular bone ; in the thorax the latter are ossified into sternal ribs, the pleurapophyses being the vertebral ribs, bearing diverging appendages, pointing back- ward, which serve to unite the ribs and to render the thorax more solid ; the haemal spines of the anterior thoracic segments are devel- oped into the broad sternum characteristic of birds, with its keel on the median line large in proportion to the powers of flight. The sacral region is greatly developed, both in the extent and in the number of bones firmly uni- ted to form it, and in its enormous pleurapo- physes, especially the ilium ; the sacrum in- cludes some of the last dorsal, the lumbar, the sacral, and even some df the caudal vertebrae as limited in the reptile skeleton ; after the sacrum come five or six caudal, more or less united, the last compressed laterally and di- rected upward. The pelvis has only two hae- mapophyses, the pubis and the ischium, not united on the median line, except in the ostrich for the former and the nandou (rhea) for the latter, the rule being that the pelvis of birds is open below. The diverging appendages of the scapular and pelvic arches, or the anterior and posterior limbs, agree in having only tw< bones in the carpus and tarsus, and th united in the metacarpus and metatarsus, sup-