NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 469 



the central nervous system, and thus the ganglia of the dorsal roots are formed. 

 The olfactory nerve alone retains its primitive condition. 



Whether there is any actual continuity, of the nature of that presumed to exist in a nerve 

 network, in the case of the neurones forming the nerve centres of the vertebrate, has been 

 somewhat disputed. The evidence is of a very doubtful character, and the relation by contact 

 with the interposition of a " synaptic membrane," as it is called by Sherrington (1906, pp. 15-17), 

 becomes marked and predominant. 



When the axone is cut off from its cell body, it undergoes a process of 

 degeneration, as a part of any cell, except that containing the nucleus, does. Owing 

 to the large number of long neurones in the vertebrate, section of their nerve centres 

 results in extensive degeneration, contrary to what obtains in animals like worms. 

 The degenerations following known, localised injuries form an important method 

 of investigating the manner of connection of one part of the central nervous 

 system with another. 



A fact to which Parker calls attention (1909, pp. 338-345) is the origin of 

 effectors, other than muscles, such as chromatophores, glands, phosphorescent 

 organs, etc., which, like that of muscle itself, appears to be independent of that 

 of the central nervous system and to be continued throughout the course of 

 evolution. These effectors become appropriated by the nervous system, as it grows 

 in its power of control. 



Many of the facts referred to above may be made clearer by reference to the 

 diagrams of Fig. 143. 



The most striking morphological fact with regard to the vertebrate nervous 

 system, as compared with the invertebrate, is that, although the cerebral mass or 

 brain in both cases lies in front of and above the alimentary canal, the continua- 

 tion along the body spinal cord in the vertebrate, ganglionic chain, or similar set 

 of ganglia, in the invertebrate has a different relationship to the alimentary canal, 

 being dorsal to it in the vertebrate, ventral in the invertebrate. Now Gaskell 

 (1908) has brought forward a theory, supported by a large body of evidence and 

 entirely in agreement with the pre-eminence of the nervous system, according to 

 which the vertebrate nervous system is in direct continuity of descent with that 

 of the invertebrate. The ventral ganglion masses grow up, as they increase, to 

 surround the primitive alimentary canal, which finally becomes the central canal 

 of the spinal cord in connection with the ventricles of the brain. This necessitates, 

 of course, the formation of a new alimentary canal, which takes place by enclosure 

 of a space by downgrowth of the body walls ventrally. There is really no more 

 difficulty in this, as Gaskell points out (1908, p. 58), than in the production of a 

 new respiratory system in the passage from fish to land amphibians and less 

 difficulty than in supposing that a new nervous system was developed. The way 

 in which this view accounts for many curious features in the erabryological develop- 

 ment of the vertebrate nervous system can only be appreciated by consultation of 

 Gaskell's book. 



STRUCTURE AND PROPERTIES OF THE NEURONE 



We may next proceed to consider, briefly, some facts as to the general structure 

 and function of the elementary constituents which make up the nervous system, 

 omitting, for the present, the question of the nerve network. 



One of the most striking characters of the neurones, at all events in the higher 

 vertebrates, is that, contrary to the cells of other organs, the whole of those which 

 the adult animal is to possess are present at birth, gradually taking on functional 

 activity. There is no evidence of any regeneration after destruction or death of 

 any individual neurone. Although there are great varieties in detail, especially in 

 size and shape, in the neurones of different function, they all consist of a nucleus 

 surrounded by cytoplasm, and have an unbranched process which may, however, 

 divide peripherally the axone ; they have also branching processes, the dendrites, 

 which also communicate with those of other neurones, as mentioned above. 



Certain observers, by examination of nerve cells fixed and stained in various 

 ways, have shown that, under these conditions, there are two obvious structural 



