THE RABBIT. 



97 



3. Dorsal and Ventral Nerve-roots. Each nerve arises 

 by two roots a dorsal and a ventral root (often called 

 posterior and anterior after human anatomy). The dorsal 

 root consists entirely of afferent fibres, and is hence also 

 called the sensory root ; while the ventral contains only 



. 



Fig. 58. DIAGRAM OF THE SPINAL CORD, DORSAL AND VENTRAL NERVE-BOOTS, 

 SPINAL AND SYMPATHETIC GANGLIA. 



The thick black lines represent nerve-fibres. 



efferent fibres, and is called the motor root. These roots 

 soon unite, and the combination immediately splits up into 

 three branches, all of which probably contain fibres of both 

 kinds : these branches are a small dorsal branch to the 

 region of the back, a large ventral branch to the main region 

 of the body-wall (so much larger than the other two that it is 

 commonly called the spinal nerve), and a visceral branch, or 

 " ramus communicans," to which we shall return presently. 

 There is another difference between the two nerve-roots 

 besides the difference in the direction in which they convey 

 impulses. On the dorsal root there is a swelling, which on 

 microscopic examination- is found to contain unipolar nerve- 

 cells: Any such collection of nerve-cells outside the central 

 nervous system is called a ganglion * ; these spinal ganglia 



* This definition of ganglia applies to vertebrates only. 

 ZOOL 7 



