208 MANUAL OF ANIMAL BIOLOGY 



important glands with ducts also form hormones and other sub- 

 stances which are passed directly into the blood stream. The 

 liver, for example, gives off bile which passes through the hepatic 

 ducts into the intestine, but the liver also secretes urea and sugar 

 directly into the blood stream. Again, the pancreas secretes 

 through its duct certain important digestive enzymes into the intes- 

 tine, but the pancreas also secretes a hormone, insulin, into the 

 blood, which influences the sugar metabolism of the body. Dia- 

 betes in Man is due to a lack of insulin. It has recently been 

 artificially isolated from the pancreas of various animals and is 

 now being used in the treatment of diabetes. Finally, the gonads 

 — both ovaries and testes — secrete certain hormones directly 

 into the blood which have a very specific action, particularly on 

 the development of certain structural features which serve to 

 characterize the two sexes. 



7. Nervous System 



We now come to a consideration of the vertebrate nervous sys- 

 tem. Previous discussions of the nervous system in various 

 Invertebrates have emphasized the paramount importance of this 

 specialized irritable tissue for the reception of environmental and 

 organic stimuli and for the proper coordination of the various 

 parts of the organism in response to stimuli received. These 

 basic functions are, of course, common to the vertebrate nervous 

 system and, in addition, the higher functions of memory and 

 intelligence come more and more into prominence and reach their 

 climax in the Primates. ( W. pp. 198-217.) 



The nervous system of the Frog can be separated into the follow- 

 ing components : (i) central nervous system. This consists 

 of the brain and spinal cord. The latter lies dorsal to the 

 alimentary tract in a special cavity, the neural canal, of the 

 vertebral column, (n) peripheral nervous system. This con- 

 sists of the paired cranial and spinal nerves which arise in the 

 central nervous system and run to all parts of the body, (hi) au- 

 tonomic, Or SYMPATHETIC, NERVOUS SYSTEM. This Consists 



primarily of a pair of ganglionated cords lying close to the dorsal 

 body wall, (iv) sense organs. These are the specialized end 

 organs of the peripheral nervous system, adapted for receiving 

 various types of stimuli from the external environment. ( W. fs. 

 107, 142.) 



