74 THE NERVOUS SYSTEM OF THE FROG 



iii. The pituitary body is a flattened ovoid sac, lying 

 behind, and continuous with, the tuber cinereum. 

 It is almost certain to be left behind in the skull 

 on removing the brain, in which case the infundi- 

 bulum will be seen torn across. 



iv. The crura cerebri are two dense white columns of 

 nervous matter, lying at the base of the optic 

 lobes, and partly hidden by the pituitary body : 

 they serve to connect the hemispheres with the 

 medulla and spinal cord. 



v. The ventral fissure of the brain is a median longi- 

 tudinal groove on the ventral surface of the hinder 

 part of the brain : it is continuous with a similar 

 groove on the ventral surface of the spinal cord. 



II. The Spinal Cord. 



The spinal cord is a somewhat flattened band, presenting 

 brachial and lumbar enlargements opposite the points of origin 

 of the nerves for the fore and hind limbs respectively, and 

 slightly constricted between these two points. About the level 

 of the sixth or seventh vertebra the cord narrows rapidly to 

 form a fine thread, the filum terminale, which extends back into 

 the canal of the urostyle. 



The tubular character of the spinal cord is best seen on 

 making transverse sections of it. (See p. 85.) 



B. The Peripheral Nervous System. 



1. The Spinal Nerves. Ten pairs of nerves arise from the 

 sides of the spinal cord ; each nerve arising by two roots, a 

 ventral or " anterior," and a dorsal or " posterior," which unite 

 at their point of exit from the vertebral canal through the 

 intervertebral foramen : just before their union the dorsal root 

 bears a ganglionic swelling. 



Within the vertebral canal the roots of the anterior spinal 

 nerves run nearly transversely outwards, so as to leave the canal 

 opposite their points of origin from the spinal cord. The roots 

 of the middle and posterior nerves, owing to the vertebral 

 column being of greater length than the part of the cord 

 belonging to it, pass obliquely backwards to their points of exit : 

 and in the case of the hindmost nerves, the roots run backwards 



