NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 485 



We see, then, that the autonomic system is in no sense an independent central 

 nervous system, but an outflow from the cerebrospinal system, distinguished by 

 its connection with neurones outside the central nervous system and by its 

 formation of peripheral plexuses or networks at the places of its distribution. 



Auerbach's plexus in the intestine has, prima facie, more claim to be called 

 an independent nervous system. Reflexes can be obtained from it, whereas no true 

 reflexes are given by any peripheral ganglion. But, if Gaskell's view of the recent 

 origin of the alimentary canal of the vertebrate be correct, Auerbach's plexus can 

 scarcely be the remains of a primitive peripheral nervous system. It has been 

 pointed out above that it has rather the characters of a synaptic system, and the 

 work of Miss Abel (1909) is of importance also in the decision of the question. 

 She investigated the embryology of this system in the bird and came to the 

 conclusion that its development is secondary to that of the cerebrospinal system. 

 This statement applies also to the whole of the sympathetic system, which shows 

 itself to be derived from the central nervous system, being produced by migration 

 of cells from the spinal cord, some of which pass downwards to the intestinal wall 

 and there form the plexuses of Auerbach and of Meissner. It will be noted that, 

 whereas the somatic nerves are formed by axones growing out from cells in the 

 central nervous system, the autonomic system is formed by chains of cells growing 

 out from the same system and forming axones subsequently. 



The system just described is, as will be noticed, entirely efferent. Now, many of the nerves 

 of the sympathetic system, such as the splanchnics, contain a considerable number of sensory 

 fibres. These are not true sympathetic fibres, since they have their trophic centres in the dorsal 

 root ganglia and simply take their course in the sympathetic rami. 



Certain special neurones have been described by Dogiel (1908, p. 133) as passing from the 

 sympathetic chain to end in networks around some of the cells of the dorsal root ganglion, but 

 their nature is uncertain. 



SUMMARY 



The object of a nervous system is to bring any part of an organism into 

 relation with any other part, without the necessity of direct nervous connections 

 from every part to every other part. It is like a telephone exchange, where each 

 subscriber has a central terminal, which can be put into connection with that of 

 any other subscriber. In the nervous system, however, the channels which bring 

 in messages from parts of the body (afferent fibres, coming from sense organs) are, 

 as a general rule, different from those fibres (efferent) which convey messages 

 outwards to organs in the body, which organs are thus caused to perform some 

 kind of action (effectors). As Pavlov has pointed out, something must be sacrificed 

 in the telephone system, since the same subscriber cannot speak to more than one 

 other subscriber at the same time. The arrangements of the central nervous 

 system are more efficient than this, since the same afferent fibre can at times be 

 connected up with several efferent fibres. 



There are thus two aspects under which the nerve centres can be studied. 

 The one is, for the most part, morphological and consists in the following out of 

 the tracts of fibres which connect its various parts together. The other consists 

 in the investigation of the means by which functional connection is established for 

 the performance of different co-ordinated actions. * 



There is reason to suppose that distinct effectors, as muscle cells, made their 

 appearance in the course of evolution previously to nervous tissue. The receptor, 

 in order to increase the sensitiveness to outer agencies, appears next in close 

 connection with the effector and, as it becomes necessary for effectors at greater 

 and greater distances from the receptor to be acted on, this receptor cell is pro- 

 longed in the form of a nerve fibre. Later, an adjuster cell is formed between the 

 receptor and effector, giving the opportunity of connecting up various receptors and 

 effectors together. 



The constituent cells of the lowest central nervous systems, as that of the 

 jelly-fish, seem to be in direct protoplasmic continuity with one another, forming a 

 true network. But, very early in evolution, we find that this mode of connection 



