If I STORY OF THE INVOLUNTARY NERVOUS SYSTEM 3 



which correspond to the infra-oesophageal nerves in the inverte- 

 brate, are divisible into cranial and spinal segmental nerves, 

 supplying motor fibres to the voluntary muscles of the cranial 

 and spinal segments respectively. 



I pointed out in 1889 thatMhe spinal and cranial segmental 

 nerves differed in that the former arose from the central nervous 

 system by two roots, ventral and dorsal, but the latter from 

 three roots, ventral, lateral, and dorsal, and that the cranial 

 region must be looked upon as more primitive than the spinal ; 

 so that the three-root system was more likely to give a clue to 

 the segmental arrangement in the invertebrate ancestor than the 

 two-root system. The work of V. Wijhe has given an explan- 

 ation of the lateral root ; for he has shown how in the embryo 

 of the selachian the mesoblast in this cranial region has a double 

 segmentation, and forms a dorsal and ventral series of segments 

 out of which the muscles are developed. The muscles developed 

 from the dorsal series are the eye muscles, and those supplied by 

 the hypoglossal nerve. Those developed from the ventral series 

 are the muscles supplied by the motor part of the trigeminal, the 

 facial, the glossopharyngeal and the vagus, that is to say the 

 muscles concerned with mastication and respiration. This double 

 segmentation has been called respectively mesomeric and branchio- 

 meric ; but because the latter includes the segments concerned 

 with mastication, which have nothing to do with respiration, I 

 called the ventral segmentation the splanchnic or visceral segmen- 

 tation in contradistinction to the dorsal, which I called the 

 somatic segmentation. 



In my book on the "Origin of Vertebrates" I have put 

 forward my conceptions of the manner in which the voluntary 

 muscles of vertebrates arose from the corresponding muscles of 

 invertebrates. 



In the invertebrate (Limulus, Scorpion, etc.) the animal is 

 divided into segments, which may be considered as forming a 

 double segmentation, that of the body and that of the appendages, 

 so that the voluntary musculature in these animals falls naturally 

 into two groups, (i) the body musculature, (2) the appendage 

 musculature. The body or somatic musculature forms in all 

 cases the great longitudinal muscles and the dorso-ventral 

 muscles. In the vertebrate the longitudinal muscles are found in 

 the trunk region and are divided segmentally to form the 



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