THE FROG AND VERTEBRATES IN GENERAL 211 



and particularly in Man. The study of the brain of the Cat or of 

 the Sheep gives a good idea of the structure of the mammalian 

 brain. (W. f. 141, E.) 



B. The Spinal Cord. The study of a section of spinal cord 

 shows it to be roughly circular in outline, but with a considerable 

 dorso-ventral flattening. It is covered by two membranes: an 

 outer, the dura mater, and an inner, the pia mater. The spinal 

 cord proper consists of an outer portion, the white matter, which 

 is composed largely of medullated nerve fibers running in a direc- 

 tion parallel to the cord, and a central portion, the gray matter. 

 In the layer of white matter there are two clefts ; one on the dorsal 

 side of the cord (dorsal fissure) and one on the ventral side of the 

 cord (ventral fissure) which is somewhat deeper. (W. f. 143.) 



The gray matter, which is surrounded by the white matter, 

 really forms the greater portion of the cord. It extends both dor- 

 sally and ventrally into the white matter in such a way that it 

 forms a pair of dorsal, and a pair of ventral, horns. In the 

 center of the gray matter and, therefore, in the center of the cord, 

 is the central canal lined by epithelial cells. The gray matter 

 contains great numbers of nerve cells, or neurons, with long proc- 

 esses, the nerve fibers. The latter extend into the white matter 

 of the spinal cord and, finally, into one of the connecting spinal 

 nerves as indicated on page 213. 



II. PERIPHERAL NERVOUS SYSTEM 



A. Cranial Nerves. There are ten pairs of cranial nerves in 

 the Frog which arise from various regions of the brain as follows. 



1. Olfactory Nerves. These arise in the olfactory lobes. They 

 pass anteriorly and somewhat laterally and innervate the olfactory 

 sensory cells lining the nasal chambers. (W. f. 142, 01.) 



2. Optic Nerves. These arise in the diencephalon of the brain 

 and then form the optic chiasm a which has been noted above. 

 From the chiasma one nerve runs to the retina of each eye and 

 innervates the sensory cells there. (W. f. 142, Op.) 



3. Oculomotor Nerves. These arise from the ventral wall of the 

 mid-brain. They run to the eyes where they innervate certain eye 

 muscles and other parts of the eye as well. 



4. Trochlearis Nerves. These arise posterior to the optic lobes on 

 the dorsal side of the brain and run to the eyes where they inner- 

 vate one pair of the eye muscles. 



