x SPINAL AND CEREBRAL NERVES 163 



Origin of the Spinal Nerves. The mode of origin of the 

 nerves from the spinal cord is peculiar and characteristic. 

 Traced towards the cord, each nerve is found, on reaching 

 the intervertebral foramen from which it emerges, to divide 

 into two a dorsal root which springs from the dorsal, and 

 a ventral root which arises from the ventral region of the 

 cord (Fig. 52, dr^ vr). The dorsal root is distinguished 

 from the ventral by being dilated into a ganglion (gn). In 

 Fig. 51 these ganglia lie hidden within certain calcareous 

 bodies (C) in this region. 



n.sp. 



./ 

 w.m ^^^^^ 



c/t. 



FIG. 52. Transverse section through the vertebral column and spinal cord of the frog, 



to show the mode of origin of the spinal nerves. 



c. c. central canal ; en. centrum ; d. f. dorsal fissure ; d. m. dura mater ; d. r. dorsal 

 root ; g. m. grey matter ; gn. ganglion of dorsal root ; n. a. neural arch ; n. sp. 

 neural spine ; /. in. pia mater (the reference line .should point to the margin of 

 the cord) ; t. nerve trunk (ventral branch) ; t '. dorsal branch ; Tr. pr. transverse 

 process ; v. f. ventral fissure ; v. r. ventral root ; w. m. white matter. (After 

 Howes.) 



Cerebral Nerves. There are ten pairs of cerebral nerves, 

 some of which are purely sensory, some purely motor, some 

 mixed. 



The first or olfactory nerves (Fig. 49, /) arise from the 

 olfactory lobes, and pass through the holes in the trans- 

 verse partition of the girdle-bone. Each is distributed to 

 the mucous membrane of the nasal sac or organ of smell 

 of the same side, and is purely sensory. 



The second or optic (//) is a large nerve which springs 

 from the ventral surface of the 'tween-brain. At their 



M 2 



