366 COMPARATIVE ZOOLOGY OF CHORDATES 



into the central nervous system and conveys the impulse 

 thither. There is, therefore, no ganglion on the dorsal root 

 of the nerves of Amphioxus, nor on the olfactory nerve in any 

 vertebrate.) 



After running into the central nervous system through the 

 dorsal root, the afferent fibres terminate and make synaptic 

 connexions with other neurons. Now the neurons in the 

 spinal cord have their cell-bodies in the grey matter which is 

 central, while the surrounding white matter is made up of the 

 axons (fibres) which pass up and down the cord to higher or 

 lower levels. The grey matter of the cord can be separated 

 into four longitudinal regions on each side. The most dorsal 

 strip is where the fibres of somatic afferent neurons terminate. 

 Beneath this is the place where the visceral afferent neurons 

 end. Under this again is the region which contains the cell- 

 bodies of the efferent visceral neurons ; and lastly the most 

 ventral part of the grey matter contains the cell-bodies of the 

 efferent somatic neurons. Thus the dorsal half of the spinal 

 cord is related to afferent and the ventral half to efferent fibres. 

 As will be seen later, this arrangement is also the fundamental 

 plan on which the brain is built. 



The axons of the efferent neurons run out of the spinal 

 cord through the ventral root. The somatic efferent neurons 

 go straight to the striped voluntary muscles of the body- wall, 

 and to the muscles of the limbs (or fins) and end in them. All 

 muscles which are innervated direct in this way by ventral 

 roots are somatic, striped, voluntary muscles derived from the 

 segmented myotomes. On the other hand, the visceral 

 efferent fibres leave the mixed nerve by the ramus communi- 

 cans, and end in the sympathetic ganglia. There they make 

 synaptic connexions with other neurons which run to the 

 smooth muscles of the viscera and form the sympathetic 

 (autonomic) nervous system. The sympathetic system will 

 be dealt with in greater detail below, but it may be noticed 

 now that the visceral efferent fibres belonging to this system 

 never run all the way to the smooth muscle or gland. There 

 is always another neuron intercalated in the circuit, and 

 carrying the impulses on from the sympathetic ganglion. The 



