NERVOUS SYSTEM 



KllKI 



vesicles being formed by expansion at the first section of the 

 spinal canal. The cerebral vesicles are : the prosencephalon, 

 thalamencephalon, mesencephalon, epencephalon and metence- 

 phalon. 



These vesicles at first form a continuation of the longitudinal 

 axis of the spinal cord, but subsequently stand at an angle 

 thereto and remain connected with one another both durine 



o 



their original form and subsequently to their transformation 

 into the different cerebral sections (ventricle, cavity of the brain)- 

 The central canal of the spinal cord, which is developed from 

 the posterior part of the primitive spinal 

 canal, is similarly connected with the brain. 

 In the lower classes of Vertebrates the 

 medulla spinalis often exceeds the brain 

 in bulk, but decreases in proportion as 

 the brain develops. The centre of each 

 half of the spinal cord consists of grey 

 nerve matter with lateral processes (cor- 

 nua) directed forwards and backwards (see 

 Fig. 78). The peripheral nervous system FIG. 78 

 originating in the spinal cord is arranged 

 according to the vertebrate articulation of 

 the body ; from each vertebra springs a 

 pair of nerves each consisting of two roots : 

 an anterior (motor) and a posterior (sen- 

 sory) nerve which unite together and form 

 one trunk, dividing afterwards into two 

 branches, a dorsal and a ventral nerve. 



Each of the extremities is supplied with a nerve plexus 

 formed of a number of anterior spinal nerves ; for the anterior 

 extremities there is the cervical plexus and the brachial plexus, 

 and for the posterior extremities the lumbar, sciatic and 

 brachial plexuses. There where the spinal nerves spring from 

 the spinal cord protuberances appear in the latter, in the fishes 

 to a slight degree but increasing in the higher Vertebrates (cer- 

 vical, thoracic and lumbar swellings) (see Fig. 79)- 



A peculiarity possessed by man in common with the Am- 

 phibians, Insectivora and Cheiroptera is the disappearing of the 

 posterior section together with the formation of the so-called 



Fmp 



Transverse sec- 

 tion of medulla spin- 

 alis from the region 

 of the back. Twice 

 natural size. (Thom, 

 Zoologie.) Fma, an- 

 terior ; Fmp, posterior 

 longitudinal sulcus. 

 Grey matter with 

 cornua enclosed with- 

 in white. Dark point 

 represents spinal 

 canal. 



