2 THE INVOL UNTAR Y NER VO US S YSTEM 



the central nervous system unites the two ; this I shall call 

 throughout this book the "connector element ". 



When these three neurons belong to the same segment, the 

 reflex may be termed a primary or segmental reflex ; and the 

 connector neurons concerned in such reflexes may be called 

 primary connector neurons (Fig. 2, A). 



A connector neuron does not necessarily communicate 

 directly with an excitor neuron but may only connect after a 

 series of communications with other connector neurons, which 

 thus form a series of relays between the receptor and excitor ele- 

 ments. By means of this relay system of connector neurons the 

 higher centres of the nervous system are brought into harmony 

 with the segmental mechanism. The connector neurons of these 

 relay systems may be called secondary connector neurons. 



The axon of a connector neuron cannot be called a motor or 

 a sensory nerve fibre ; I propose therefore to call it simply a con- 

 nector fibre. When such 'connector fibres are grouped together 

 in definite parts of the central nervous system, they are called 

 tracts, which are designated as ascending or descending according 

 to the position of the nerve cells, of which they are the axons. 

 The great characteristic of these connector nerve fibres is the 

 formation of collaterals, by means of which each nerve fibre 

 connects with more than one neuron. 



In all cases the receptor neurons of the voluntary system are 

 found in the posterior root ganglia both in the cranial and spinal 

 regions, and the excitor neurons in the groups of nerve cells 

 found in the central nervous system, which form the motor 

 centres and give origin to the motor nerves of the voluntary 

 muscles. These motor cell groups are not situated in the same 

 position in all parts of the central nervous system. We have every 

 reason to believe that the central nervous system of the verte- 

 brate indicates a derivation from a segmentally arranged nervous 

 system, such as is found in the higher forms of invertebrates, and 

 the nerves which supply these segments form a succession of 

 segmental nerves, which commence just posterior to the infundi- 

 bulum and form a great group of nerves extending to the end 

 of the body. They may therefore be termed infra-infundibular, 

 in contradistinction to the nerves of special sense, olfactory and 

 optic, which arise from the supra-infundibular region of the central 

 nervous system. These infra-infundibular segmental nerves, 



