Page:The American Cyclopædia (1879) Volume XIII.djvu/442

 428 PHILOSOPHICAL ANATOMY istence of four organs of sense in the head, au- ditory, gustatory, optic, and olfactory ; on this principle there would seem more reason for the admission of only three. In the first place, there are three cerebral vesicles, corresponding to what become cerebellum, optic lobes, and cere- bral hemispheres, or according to some anato- mists the medulla oblongata may be substituted for the cerebellum ; in the next place, there are only three special senses in the head, hearing, seeing, and smelling, taste being a compound sense, made up of smell and touch ; the flavor of substances we get from the sense of smell, as the result of a common cold in the head or of artificially preventing the entrance of air by holding the nose sufficiently shows ; the rest of the sense of taste is a delicate modification of the sense of touch, placed for protective pur- poses at the commencement of the alimentary canal ; it is absent in many of the lower verte- brates, and has no more claim to be reckoned among the special senses than similar modifi- cations of the sense of touch in the wing of the bat or in the genital mucous membrane ; the origin, development, and mode of distribu- tion of the gustatory nerve, which is only a branch of the fifth pair, moreover, are not such as belong to special sense organs. There are also only three sense capsules in the head, the petrous portion of the temporal bone, the scle- rotic in the eye (as in the tunny), and the eth- moid for the sense of smell. As each vertebra of the trunk corresponds to a spinal nerve, there ought to be, according to the view here maintained, three pairs of nerves in the head ; excluding the three special sense nerves, the first pair of cranial nerves would be made up of the motores oculorum, pathetic, external mo- tor of eye, and the facial (or the third, fourth, sixth, and seventh) for the motor portion, and the fifth or trif acial for the sensitive portion ; the second pair of cranial nerves has the glos- so-pharyngeal and spinal accessory for its motor portion and the par vagum for the sensitive ; the third pair of cranial nerves is the hypoglos- sal, which, though all motor in man, in rep- tiles (frogs) has the sensory ganglion of an ordinary spinal nerve; in the same manner in reptiles the seventh is seen to belong to the first, and the glosso-pharyngeal and spinal ac- cessory to the second series. We have, then, three cerebral vesicles, three special senses, three sense capsules, and three pairs of cranio- spinal nerves, which would seem to indicate three cranial vertebrae, with a rudimentary nasal or other vertebral bodies in front, with- out nerves belonging to them, corresponding to the coccyx posteriorly. There is no regu- larity in the manner in which the nerves, both spinal and cranial, come out of the vertebral canal ; the second cranial nerve comes out at the jugular foramen, between the occipital and parietal vertebrae ; some of the nerves of the first pair make their exit from the cranium by the foramina rotundum and ovale, and some by the sphenoidal fissure, that is, both through the second and between the first and second ; in the human spine the nerves come out be- tween the vertebrae, but in the dorsals of many maminals they pierce the middle of each verte- bra. As to the haemal arches, Prof. Owen finds it very difficult so to divide his tympanic series as to get the hyoid arch and lower jaw as ap- pendages to the second and third vertebras; there is considerable doubt as to whether the diverging appendages of- the cranial vertebras are as yet properly determined. Admitting three cranial vertebras, with a rudimentary fourth or nasal centrum, let the occipital seg- ment claim the scapular arch ; the lower jaw may be appended to the parietal and the upper jaw to the frontal segment, the hyoid arch, as Oarus and others maintain, being placed with the tracheal rings in the splanchnoskeleton ; or, leaving the hyoid arch in the endoskeleton (which is probably more correct), and pertain- ing to the parietal segment, the upper and low- er jaws may be made the double haemapophy- sial appendages to a single rib, as the ischium and pubis are to the ilium. There are some facts favoring the latter view, such as the de- velopment of both jaws from a single arch ; in the cyclostome fishes the mouth is an arch, in which it is hard to say which part constitutes the upper and which the lower jaw ; in myx- ine there is no under jaw, the inferior portion of the mouth being made up of the anterior part of the tongue bone. Closely interwoven with embryology, philosophical anatomy, and zoological classification, is the idea first enun- ciated by Carus, and afterward extensively de- veloped by Prof. J. D. Dana, that in the higher groups of animals the more important parts of the structure are centralized in the head called by Dana cephalization. He first applied this idea as a principle of classification in his "Report on the Crustacea of the Exploring Expedition " in 1852 ; he afterward very fully explained his system in the " American Jour- nal of Science " for 1856, 1858, 1863, 1864, and 1866, inventing a great number of new terms which cannot be introduced here ; those inter- ested can consult the above mentioned volumes. The fundamental idea is the higher centrali- zation or cephalization of the superior grades of animals, and the less concentrated central forces of the inferior; in the higher groups the centralization contributes to the head func- tions, or those of the senses and the mouth ; as we descend in the animal scale, the head loses one part after another for purposes of locomotion. This cephalization is manifested in the nervous system and in the members of the body. Intimately connected with this is the element of size, which is an important one in animal structure ; in the Crustacea, especial- ly, diminution of size generally accompanies a departure from the centralization of the or- gans in the head, and the increased develop- ment of the thoracic and abdominal regions. The same relation of cephalic predominance is as apparent in the embryological development