80 AN INTRODUCTION TO ZOOLOGY 



relative positions are reversed, for here it is the white matter that is 

 outside and the grey matter inside. The transverse section of the 

 cord presents a very characteristic appearance. It is approximately 

 trapezoidal with well-rounded corners. On the dorsal side is a very 

 shallow groove, the dorsal fissure, and on the ventral side is a well- 

 marked fairly deep furrow, the ventral fissure. The grey matter is 

 arranged within the white in such a way that its outline is somewhat 

 similar, only exaggerated, and it is possible to recognise two dorsally 

 situated blunt horns or dorsal cornua, and two ventral ones, the 

 ventral cornua. In these, particularly in the latter, will be seen the 

 bodies of large nerve cells. Near the middle of the section will be 

 seen the small canalis centralis lined by a characteristic neural 

 epithelium. In addition to the actual nerve cells, there are present 

 in the spinal cord certain supporting elements known as the 

 neuroglia cells. 



Peripheral Nervous System. 



The frog possesses ten * pairs of cranial nerves coming 

 off from the brain, and ten pairs of spinal nerves coming from 

 the spinal cord. Each nerve is made up of a large number 

 of fibres bound together, and breaks up into smaller and smaller 

 bundles as it passes away to the tissues. A nerve fibre is only 

 capable of conveying a nervous message or impulse in one 

 direction, consequently one set of fibres takes messages to the 

 central nervous system, and these are termed afferent or sensory 

 nerves, while the other set, taking messages away from the brain, 

 are termed efferent or are spoken of as motor nerves. It sometimes 

 happens that all the many fibres composing one nerve are of the same 

 sort, and we then use the terms " afferent " and " efferent " to describe 

 the nerve as a whole, while if it contains both kinds of fibres it is 

 called a mixed nerve. 



The first pair, the olfactory nerves, come from the front end of 

 the olfactory lobes and are sensory, being distributed to the lining of 

 the nasal cavities, i.e. the olfactory organs. The second or optic 

 nerves, the nerves of sight, are large sensory nerves arising from the 

 side of the brain beneath the optic lobes. They partly cross over 

 to the other side on the ventral side of the brain forming the optic 

 chiasma, and they run through the orbit into the eyeball, where they 

 spread out over its inner surface. Surrounding them is a muscle, 

 the retractor bulbi, which can pull the eyeball back into its orbit. 



* In addition to these ten cranial nerves there are also two others, the 

 nervus terminalis and nervus septalis, but they are very small and difficult to 

 find. They will be dealt with more fully later in considering the Dogfish and 

 Rabbit, and it is sufficient to simply notice their presence at this place. 



