CHAPTER X. 



THE CONNECTOR NEURONS OF THE INVOLUNTARY NERVOUS 



SYSTEM. 



I COME now to the consideration of the cells in the central 

 nervous system which give origin to the various outflows of 

 connector nerves the thoracico-lumbar, the bulbo-sacral and 

 the prosomatic. 



In Chapter I, I discussed the position of the motor, connector, 

 and sensory neurons in the voluntary nervous system, and pointed 

 out that they must be considered in the light of a double seg- 

 mentation, which I have called the somatic and splanchnic seg- 

 mentations. In my book on the " Origin of Vertebrates " I have 

 shown how this double segmentation follows naturally from the 

 double segmentation so characteristic of the higher invertebrates, 

 formed by the body segments on the one hand, and the appen- 

 dages on the other. The muscles of the somatic segmentation of 

 the invertebrate have formed the corresponding longitudinal and 

 dorso-ventral groups of muscles of the vertebrate ; and the 

 muscles of the appendages have become the muscles of the 

 splanchnic segmentation. The latter in the prosomatic region 

 forms the muscles of mastication, which have been derived from 

 the muscles of the masticatory appendages ; and in the mesomatic 

 region they form the muscles of respiration and the associated 

 facial muscles which are supplied by the facial nerve, and also 

 the deglutition and respiratory muscles supplied by the glosso- 

 pharyngeal and vagus nerves. These mesosomatic appendages, 

 which were originally free respiratory appendages, as in Limulus, 

 became internally situated, as in the scorpion group, and formed 

 the commencement of the new alimentary canal of the vertebrate, 

 which is unique among alimentary canals in the marked segmen- 

 tation of its anterior end. The branchial chamber, so formed, 

 was originally closer to the anal end of the body, as can be in- 

 ferred from the appearances of such ancient forms as Bothriolepis, 



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