CHAPTER IV 



THE FROG: NERVOUS SYSTEM AND 

 SENSE ORGANS 



In the nervous system of the frog there may be recognised 

 two main parts — the cerebrospinal system, con- 

 System 8 : nected with the organs of sense and the 

 General voluntary muscles, and the sympathetic system, 

 connected with the viscera and blood vessels. 

 The cerebro-spinal system comprises the central nervous 

 system, or cerebro-spinal axis, composed of the brain and 

 the spinal cord, and the peripheral nervous system, con- 

 taining the cerebro-spinal nerves and certain knots of nerve 

 cells upon them, known as their ganglia. There are ten 

 pairs of cranial nerves arising from the brain, and the 

 same number of spinal nerves. The sympathetic system 

 also consists of nerves and ganglia. 



The spinal cord is an elongated, subcylindrical structure, 

 ai cord ty m S ' n tne vert ebral canal of the backbone. It 

 is somewhat flattened from above downward, 

 tapers to a fine thread, the filum terminate, in the urostyle, 

 and swells somewhat in the regions of the limbs. A trans- 

 verse section (Fig. 57) reveals the fact that it is traversed by 

 a central canal, which ends blindly behind, but in front is 

 contirtuous with cavities in the brain. It is composed of 

 nervous tissue enclosed in a connective tissue sheath, the 

 pia mater, which, along the dorsal and ventral middle lines, 

 passes in to some depth as the dorsal and ventral fissures. 

 The nervous tissue is of two kinds, a white matter outside 

 and a grey matter around the central canal. In transverse 

 section the grey matter extends as dorsal and ventral horns 

 on each side. 



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