22 



THE NERVOUS SYSTEM 



tube associated with segmental nerves. In true vertebrates the anterior end of 

 the neural tube becomes irregularly enlarged to form the brain, while the pos- 

 terior end remains less highly but more uniformly developed and forms the 

 spinal cord. 



The primary motor nerve-cells of vertebrates resemble very closely those of 

 invertebrates in being located within the central nervous system and in send- 

 ing motor nerve-fibers to the muscles (Fig. 31). The primary sensory cells lie 

 outside the central system, as in invertebrates. Those for smell are located in 

 the olfactory epithelium. But all others have migrated centrally along the 

 sensory fibers, and now send one process toward the periphery and another into 



bra 



Fig. 4. Transverse section of the ventral nervous cord of Sigalion: bm, Basement mem- 

 brane; c, cuticula; e, epidermis; gc, ganglion-cells; n, nerve-fibers and neuropil; s, space occupied 

 by vacuolated supporting tissue. (Parker, Hatschek.) 



the central system. The relative positions of these cells in the annelid, mollusc, 

 and vertebrate are illustrated in Fig. 5. In the latter the sensory cells are aggre- 

 gated into masses known as the cerebrospinal ganglia, which are associated 

 with peripheral nerves and are usually placed near the point of origin of these 

 nerves from the brain or spinal cord. A comparison of Figs. 3 and 31 will show 

 a striking similarity between the simple reflex arc in the earthworm and in man. 

 If space permitted we might trace the development of the central nervous sys- 

 tem in some detail, but perhaps enough has been given to suggest that the 

 nervous system of man represents the culmination of a long process of evolu- 

 tion which began with a simple sensory mechanism like that of the sea-anemones. 

 We shall be concerned with a study of the vertebrate nervous system, almost 



