NERVOUS SYSTEM 



«.I 1 



coming from the dorsal columns of grey matter, remain distinct 

 from the ventral roots, which come from the ventral columns, 

 but in craniates the two roots of each spinal nerve join to form 

 a mixed nerve. The cranial roots, as we have seen, remain 

 separate. The dorsal root has a ganglion outside the spinal cord, 

 the ventral root has one inside. The mixed nerve generally 

 divides into a dorsal and a ventral ramus which supply the 



epidermis 



free nerve 

 ending 



%- neuromast 



taste-bud 



myomere 



wall of gut 



free rierve 

 ending taste- 

 bud 



Visceral 

 muscle 



gland 

 cells 



Fig. 471. — Diagram showing the central origin from the medulla of the nerve 

 components, and their peripheral distribution. — From Goodrich, Studies on 

 the Structure and Development of Vertebrates, 1930. Macmillan, London. 



appropriate parts of the body. The ventral rami of several 

 segments in the pectoral and pelvic regions combine to form a 

 plexus for each limb. The pattern of this is usuall\- highly 

 characteristic for the species. 



The fibres which make up the nerves are afferent or sensory 

 if they carry impulses into the central nervous system, efferent 

 or motor if they carry them out. They are also divided according 

 to the part of the body which they serve, so that there are four 

 main types (Fig. 471). Somatic sensory fibres convey impulses 

 which are initiated by the outside world (the exteroceptive held) 



