Page:The American Cyclopædia (1879) Volume XIII.djvu/439

 PHILOSOPHICAL ANATOMY 425 The autogenous elements enclose generally foramina which form canals in the vertebral chain ; the most constant and extensive canal is that marked n, above the central body, for the lodgment of the spinal cord or neural axis, composed of the laminae hence called neurapo- pJiyses ; the second canal, marked A, below the centrum, is more irregular and interrupted, lodging the central vessel and the great trunks of the vascular system, and is formed by the lamellae hence called hcemapophyses. On the sides of the centrum, most commonly in the region of the neck, is a canal circumscribed by the pleurapophyses or costal processes, the parapophyses or inferior transverse processes, and the diapophyses or superior transverse pro- cesses. Thus a perfect or typical vertebra, such as is found in the thorax of man and most of the higher vertebrates, and in the neck of many birds, with all its elements, presents four canals around a common centre ; in the tail of most reptiles and mammals the haemapo- physes are joined to the lower part of the centrum, protecting only the artery and vein ; but when the central organ of circulation is placed within it, the haemal arch is largely developed, as in the thorax, where the pleura- pophyses (ribs) are much elongated, and the haemapophyses (costal cartilages) are removed from the centrum and placed on the end of the ribs, the bony circle being completed by the haemal spine or sternum; the neural spine is the equivalent of the superior spinous process. He shows the fallacy of Ouvier's definition of a vertebra; the latter maintained that verte- brae have a special number of pieces arranged in a definite manner, looking more at their po- sition in the series than at their composition ; his prejudices against the vertebral theory led him into many untenable and contradictory statements and definitions. Cuvier divided the bones of the head into cranial and facial, making three annular segments of the former ; the anterior comprised the frontal and ethmoid, the middle the parietals and sphenoid, and the posterior the occipital, the temporals being in- tercalated between the occipital, parietal, and sphenoid ; he does not apply this to the lower vertebrates, in which it is most evident, nor to the face, or he would have found that these divisions do not include the same bones in all animals, the same being in one a cranial and in another a facial element; this again in- volved him in many inaccuracies and contra- dictions. Owen divides the endoskeleton of the human head into four segments, as follows, beginning behind : 1. Occipital or epencephalic vertebra, with the following composition : cen- trum (c.), the basi-occipital portion of the oc- cipital bone; parapophyses (p.) and neurapo- phvses (n.), coalesced in the lateral or con- dyloid portions, the former marked by the ridge for the rectus lateralis muscle; neural spine (n. s.), the proper occipital bone ; pleura- pophyses (pi.), the scapulae ; diverging appen- dages (d. a.), the bones of the upper extremity ; haemapophyses (h.), the coracoid processes of the scapulae ; and haemal spine (h. s.) deficient. The clavicle and first segment of the sternum, which complete the mammalian scapular arch, are the haemapophyses and haemal spine of the atlas or first cervical vertebra. 2. The parietal or mesencephalic, with c., the basi- sphenoid or posterior part of the body; p., mastoid processes ; n., greater wings of sphe- roid; n. s., parietal bones; pi., styloid pro- cesses ; d. a., greater cornua of hyoid bone ; h., lesser cornua; and h. s., body of hyoid. 3. Frontal or prosencephalic, with c., anterior body of sphenoid; p., external angular pro- cesses of frontal (post-frontals of fishes); n., lesser wings of sphenoid ; n. s., frontal bone ; pi., tympanic portion of temporal; d. a., defi- cient ; h., articular portion of lower jaw ; and h. s., dental portion of same. 4. Nasal or rhinen- cephalic, with c., vomer ; n., ossa plana of eth- moid; n. s., nasal bones; pi., palate bones ; d. a., pterygoid and malar bones, with squamous and zygomatic portions of temporal ; h., supe- rior maxillary bones ; and h. s., intermaxillaries. The splanchnoskeleton of the head consists of the petrosal and ear bones, the turbinated bones, and the teeth ; the external skeleton consists of the lachrymal bones. These four cranial vertebrae, according to the organs of sense, would be: 1, auditory, related to the organ of hearing ; 2, gustatory, with the organ of taste, whose nerve (gustatory or trifacial) pierces the neural arches of this segment or passes between it and the frontal; 3, optic, with the organ of vision between this and the nasal segment ; and 4, olfactory, with the or- gan of smell always in front. Agassiz, in the first volume of his Poissons fossiles, arguing from the fact that the cephalic extension of the chorda dor sails is arrested in the embryo fish at the region of the greater sphenoidal wings, maintained the "existence of only one cranial vertebra, the occipital, the rest of the head remaining foreign to the vertebral sys- tem ;" this is refuted by Owen. Owen also combats the idea of Oken that the head is a repetition of the whole trunk; he maintains that the jaws are not the limbs of the head, but are the modified haemal arches of the two anterior segments ; the anterior limbs are di- verging appendages of the occipital segment, and the posterior of the pelvic segment with its haemal arch, both variously displaced from their haemal arches in different vertebrates. The diverging appendages of the ribs of fishes, reptiles, and birds, arising from their posterior edge, are essentially limbs, rudimentary arms and legs, though they never become such. As the cranial segments are in number according to the cranial nerves of sense, so the develop- ment of the vertebral bodies and neural arches in the trunk depends on the junction of the nerves with the spinal cord; the condyloid foramen of the occipital bone gives passage in man to the hypoglossal nerve. The cranial bones of fishes are exceedingly complicated,