NERVE COMPONENTS 21 



ramus lateralis vagi (in the adult) from the spinal nerves, and the fact 

 that in fish (Beard, Coilson) and in Amphibia (Harrison) the 

 lateral-line rudiment grows from the head backwards in a most 

 independent manner, have led most observers to believe that the 

 organs have phylogenetically been derived from the head. It is 

 a point still unsettled. The chief functions of the organs of the 

 lateral-line system are probably equilibration and orientation. 



The sensory free nerve-endings scattered over the skin are 

 supplied by fibres belonging to the 'general cutaneous sensory 

 system,' and entering the central nervous system by the 5th, 7th, 

 9th, and 10th cranial nerves, and by the succeeding dorsal roots of 

 the spinal nerves. The fibres are related to the dorsal tracts of the 

 .spinal cord, and their prolongations in the medulla, including the 

 tuberculum acusticum. It is conjectured that the acustico-lateral 

 system is a specialised portion of this general cutaneous system. 



The nerve-fibres of a similar 'general splanchnic sensory, or 

 coramunis, system,' in the wall of the alimentary canal, enter the 

 brain by the 7th, 9th, and 10th cranial nerves and by the 

 sympathetic fibres in the dorsal spinal nerves, and terminate in 

 Clarke's column, the fasciculus communis, the lobus vagi, and 

 associated centres. To the same centres in the brain come the 

 fibres from a system of gustatory 'end-bud organs,' or 'taste- 

 buds,' distributed over the buccal and pharyngeal cavities, and 

 also, in some fishes, spreading over the outer surface of the head 

 and body. The nerves from this 'special splanchnic or end-bud 

 sensory system' reach the brain by the 7th, the 9th, and the 

 10th cranial nerves. The end-bud system is supposed to have been 

 differentiated from the splanchnic general sensory system. The 

 taste-buds situated on the surface of the body on fins (Teleostei) 

 would appear to have migrated from the endoderm ; but the reverse 

 may possibly have occurred. 



To these four sensory systems must be added the ' somatic 

 motor system,' communicating with their centres by the ventral 

 roots ; and the ' splanchnic motor system,' receiving fibres through 

 the dorsal roots of the cranial and spinal nerves, and the sympathetic 

 nerves, also from special central regions. 



Thus, it has been shown that the nervous system of the 

 Craniates can be subdivided into several distinct components, four 

 sensory and two motor, each with its own type of 'end-organ,' 

 its own set of nerve-fibres, and its own special nerve-centres 

 (Strong [428], Herrick [210], Johnston [249]). It is further con- 

 jectured that the ' general cutaneous ' and ' splanchnic ' sensory 

 systems, with their more specialised derivatives, and the motor 

 systems, may each have originally been represented in every 

 segment of the body. 



The size of nerve components in a given region is proportional 



