72 THE FROG 



largement. Behind the lumbar enlargement the cord gradu- 

 ally tapers to a point. Observe the dorsal longitudinal 

 fissure. Carefully cut down the stumps of the neural arches 

 and expose the roots of the spinal nerves. Of these the 

 most conspicuous are the second pair, in the brachial region, 

 and the seventh, eighth and ninth, in the lumbar region; the 

 others are very small and likely to be overlooked. As these 

 roots pass through the intervertebral foramina they are 

 surrounded by the dense, white calciferous bodies or so- 

 called periganglionic glands, each of which, as the latter 

 name implies, encloses a spinal ganglion. The calciferous 

 bodies are not glandular in nature, but represent extensions 

 of the endolymph space of the outer covering membrane 

 (dura mater) of the spinal cord, filled with fine granules of 

 calcium carbonate. 



Beginning at the posterior end, the divisions of the brain 

 as viewed from the dorsal aspect are : 



(a) The medulla oblongata. This may be easily recog- 

 nized by the triangular cavity on its dorsal surface. This 

 cavity is the fourth ventricle of the brain and is continu- 

 ous with other cavities (ventricles) in the brain and with a 

 central canal in the cord. The cavity of the fourth ven- 

 tricle is roofed over by vascular epithelium which is thrown 

 into numerous inturned folds forming the posterior choroid 

 plexus. 



(b) The cerebellum is the transverse fold lying in front 

 of the medulla. In the frog it is very small. 



(c) The optic lobes are two rounded masses imme- 

 diately in front of the cerebellum. 



(d) The thalamencephalon is the portion between the 

 optic lobes and the cerebral hemispheres (see below). Its 



