174 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



sends fibres which extend longitudinally in either the dorsal or the 

 lateral funiculi of the cord; the other process extends peripherally to 

 the skin or viscera, these processes constituting the dorsa l root of the 

 nerve, the ganglion forming an enlargement upon it, nearTFT? connex- 

 ion with the cord. 



The other or v entra lroot is formed by fibres which grow out in a 

 similar way from neuroblasts in the ventral horn of the cord and leave 

 it between the anterior and lateral funiculi, to extend to the muscles, 

 glands, etc. As these ganglion cells are inside the cord, there is no 

 ganglion on the ventral root. Except in the lampreys, the dorsal and 

 ventral roots unite soon after leaving the cord, the combined trunk 

 being a typical spinal nerve (figs. 154, 188). 



Physiologically the roots differ in that the dorsal roots are mainly 

 composed of sensory fibres, while the ventral roots contain only 

 motor fibres. That is, on stimulation of the parts to which they are 





Scjnatlc ynoior 

 Visceral motor 



Fig. 184. — Diagrams of spinal cords of (A) fish, and (B) man, after Johnston, 

 showing the relations of the components of the spinal nerves, dr, vtir, vsr, dorsal, 

 ventral and visceral rami of spinal nerve. 



distributed the dorsal roots and their fibres carry nervous impulses to 

 the cord — they are affgtent — while the impulses in the ventral roots 

 are carried in the opposite direction by efferen t fibres. In the latter 

 stimulation arises in the central nervous system and the impulse is 

 carried outward to the parts to which the fibres are distributed, 

 causing these to act — muscles to contract, glands to secrete, etc. 

 Hence the ventral roots are called motor roots. Their fibres are 

 without sensory functions, while sensory fibres are equally unable to 

 cause action in any peripheral par t (Bell's law)^ . 



After a longer or shorter course, each spinal nerve, formed by the 

 union of dorsal and ventral roots ; divides into three branches, each of 

 which receves both sensory and motor fibres. These are known as 

 the ramus dorsalis, ramus ventralis and ramus visceralis or intesti- 

 nalis. The first goes to the skin and muscles of the dorsal region; the 



