232 



COMPARATIVE ANATOMY 



Ectoderr, 



The spinal nerves retain the more primitive and simple 

 relations, and all show a similar arrangement along both dorsal 

 and ventral regions of the spinal cord, so that each segment of the 

 trunk possesses a 'dorsal and a ventral pair. The former consist 

 mainly of afferent (centripetal) or sensory, the latter of efferent 

 (centrifugal) or motor fibres (Fig. 176). 



Each dorsal or sensory nerve has a ganglion in connection 

 with it, while on the ventral nerves a ganglion is wanting, except 

 in the embryo of certain Fishes. The ventral nerves, which 

 mainly supply the great lateral muscles of the trunk and their 

 derivatives, arise as direct outgrowths from the spinal cord, while 

 the dorsal nerves first appear as outgrowths from their ganglia, 

 coming into connection with the cord secondarily. The ganglia them- 

 selves are developed from a neural ridge of ectoderm cells lying 

 close to the junction of the medullary cord and outer ectoderm 

 (Fig. 177). On the distal side of each ganglion, both nerve-roots 



almost always become bound 

 up in a common sheath, though 

 many facts seem to indicate 

 that in the ancestors of exist- 

 ing Vertebrates the dorsal and 

 ventral roots remained dis- 

 tinct, as in fact is still the 

 case in Amphioxus and Petro- 

 myzon. 



In addition to the somatic 

 afferent and efferent fibres, 

 which innervate the skin and 

 muscles of the dorsal and 

 lateral portions of the myo- 

 tomes, certain afferent and 

 efferent visceral components 

 also pass through the dorsal 

 and ventral nerves. The former 

 are in connection with ganglion cells of the sympathetic system, 

 situated more peripherally, and pass along the dorsal roots; 

 while the latter arise from nerve-cells in the ventro-lateral regions 

 of the spinal cord, and pass out through both the dorsal and 

 ventral roots, without, however, having any physiological connection 

 with the ganglia of the former. These supply the muscles of the 

 viscera and blood-vessels which originate from the lateral plates of 

 the mesoderm (p. 9), but do not innervate the skeletal muscles. 



The common nerve-trunk formed by the junction of the two 

 roots divides up again into a dorsal and a ventral branch, containing 

 both afferent and efferent somatic fibres, and a visceral branch, 

 containing afferent and efferent visceral fibres. The first of these 

 goes to the muscles and skin of the back, the second supplies the 

 lateral and ventral portions of the body-wall, while the visceral 



Spinal cord 



FIG. 177. DIAGRAM OF THE EMBRYONIC 

 SPIXAL CORD AND THE NEURAL 

 RIDGE FROM WHICH THE SPINAL 

 GANGLIA ARE DIFFERENTIATED. 

 (After J. S. Kingsley.) 



