336 



AN INTRODUCTION TO ZOOLOGY 



and their junction is marked by the formation of a groove. This 

 continues on down the cord as a narrow but quite deep cleft, the 

 dorsal fissure. On the ventral side is another groove, the ventral 

 fissure, not quite so deep and somewhat wider than the dorsal one. 

 As previously noted, the same meninges that we find in the brain 

 surround the spinal cord, but the arachnoid is divided into two 

 layers, separated by quite a large sub-arachnoid space, which 

 communicate on the dorsal side by a septum. 



The canalis centralis is very much reduced, and its lumen appears 

 as a tiny space situated nearer the end of the dorsal fissure than the 

 ventral. It is lined by ependymal epithelium continuous with that 

 of the brain cavities. In transverse section the cord is seen to be 

 composed of a characteristic H-shaped arrangement of grey matter 

 surrounded by white matter. The canalis centralis lies in the 

 middle of the transverse bar of the H, and has above it a band of 

 transverse fibres, the dorsal commissure, and below it a wider but 

 similar band, the ventral commissure. 



Spinal Nerves. 



Throughout the whole of its length the spinal cord gives 

 off the paired segmental spinal nerves, leaving the vertebral column 



by the intervertebral fora- 

 mina. Each arises as in 

 all vertebrates by two 

 roots, a dorsal root related 

 to the dorsal horn of the 

 grey matter, and a ventral 

 root issuing from the ven- 

 tral horn. The dorsal root 

 bears, a short distance 

 from its origin, the dorsal 

 root ganglion, almost im- 

 mediately beyond which 

 the two roots unite to 

 form a common trunk. 

 The common nerve so 

 constituted leaves the 

 vertebral column and 



gives off a small ramus dorsalis passing to the muscles and skin of 

 the dorsal body region and tiny strands to the sympathetic chain. 

 The main part goes on as the ramus ventralis to be distributed to 

 the more ventrally situated portions of the body. 



The nature of the two roots of the spinal nerves is a matter of 

 some interest. If the common trunk after the point of fusion be 



FIG. 115. Illustrating the functions of the 

 roots of the spinal nerves. From Fur- 

 neaux 



a., ventral root ; p., dorsal root. 



Divided at a. Irritated at i : no result. Irritated 

 at 2 : contraction of muscles supplied with fibres from 

 the root. 



Divided at . Irritated at 3 : no result. Irritated 

 at 4 : pain produced. 



