328 SPECIAL PHYSIOLOGY. 



It would seem, that in certain vertebrate animals, as distinguished from 

 man, more of the longitudinal fibres found in the spinal cord are commissural 

 between its several parts, or else are concerned in connecting the roots of the 

 spinal nerves with its own gray matter, whilst fewer ascend, from the nerves, 

 to the sensorial centres at the base of the cerebrum ; this has been shown at 

 least to be the case, in regard to the spinal cord of the horse. (Volkmann, 

 Kolliker. ) In accordance with this, the locomotive movements of such ani- 

 mals, are either consensual, or sensori-motor, or, as in those very low in the 

 vertebrate scale, even purely excito-motor, becoming thus gradually less de- 

 pendent on, or wholly independent of sensation ; less associated with ideas of 

 purpose, less influenced by education, and at last, perhaps, purely automatic, 

 and wholly dependent on the internal structure of the nervous apparatus con- 

 tained in the spinal system. In the higher Mammalia, and in man, however, 

 the locomotive movements are, to a greater extent, guided by sensation, and 

 are commonly regulated by proper cerebral or mental processes. It has 

 further been observed, as the result of section of the cord, that, in Birds and 

 Reptiles, the decussation of the paths of sensation in the cord is less direct, 

 and less perfect, than in Mammalia. In the Amphibia, as in frogs, the move- 

 ments performed, through the spinal cord, in the decapitated animal, are so 

 purposive, that they simulate the volitional acts of the nigher Yertebrata and 

 of man, but their marked uniformity justifies the denial to them of any voli- 

 tional quality, a view also supported by analogy, which is opposed to the sup- 

 position that psychical endowments can be manifested by the spinal cord, in 

 some Vertebrata, and not in others. 



The Cranial and Spinal Nerves. The cranial nerves, in the Yertebrata gen- 

 erally, correspond in arrangement, general distribution, and function, with 

 those which we have described in man ; but in certain cases, some of them are 

 absent, whilst others may have a more extensive, or a limited, distribution 

 and office, than they have in man. For example, the olfactory nerves are 

 absent in certain Cetacea ; and the optic nerves are wanting in the Mole, and 

 in the blind Fishes found in perfectly dark subterranean caves, as in those of 

 Kentucky. Again, the hypoglossal nerve is more complete in the cat, ox, and 

 rabbit, than in man, for it has a posterior root, with a small ganglion upon it, 

 and so, resembles exactly an ordinary perfect spinal nerve. On the other hand, 

 this nerve becomes very small in certain animals, such as Birds, in which the 

 tongue is but slightly developed ; and it is still smaller in Fishes, in which it 

 may be ranked with the spinal nerves. In certain fishes too, the fifth pair of 

 nerves has an unusually extensive distribution, sending, in particular, a large 

 branch, the so-called great lateral nerve, down the whole length of each side of 

 the body. In Fishes, also, the vagi or pneumogastric nerves, although they 

 no longer supply lungs, nevertheless send branches to the substituted respira- 

 tory organs, the gills. 



The Sympathetic Nervous System. The sympathetic nerve is well developed 

 in all the Yertebrata, and is constructed on the same plan as in man, consist- 

 ing of a double ganglionated cord, communicating at numerous points with 

 the cranial and spinal nerves, and giving off branches forwards, on which pre- 

 vertebral ganglia are found, and supplying all the viscera, its ultimate twigs 

 spreading out on the coats of the small arteries. Its offices are doubtless the 

 same as in man ; for it presides over the lowest, or vegetative system of func- 

 tions. 



The Nervous System of the Amphioxus or Lancelet. This little animal, which 

 is the lowest Fish, and therefore stands at the bottom of the whole Yertebrate 

 series, is so remarkable, as here to deserve a special notice. It is found prin- 

 cipally in the Mediterranean, on the coast of Italy ; but it has been, though 

 rarely, caught in the Atlantic, and even as far north as the British seas. It 

 is an oblong transparent animal, about three-quarters of an inch in length, 

 blunt at its anterior, and pointed at its posterior extremity, slightly flattened 

 at its sides, and provided with a thin marginal fin, extending along the whole 

 back and tail, and as far forwards beneath, as the abdomen. It has a simple, 

 short, alimentary canal, and a series of slit-like branchial openings at the sides 

 of the pharynx, fringed with gill-like processes, constituting the respiratory 

 apparatus. It has no distinct heart ; but presents, instead, a series of contrac- 



