326 



THE VERTEBRATE BODY 



strong that the proper connections are made in the spinal cord and the 

 response comes quickly and vigorously. 



The autonomic nervous system consists of two nerve trunks that run 

 down the body on either side of the vertebral column with branches run- 

 ning out to those organs which are controlled automatically. They 

 regulate the rate of heart beat, rate of respiration, secretion of digestive 

 enzymes, movement of food through the intestine, the dilation and con- 

 traction of the pupils of the eyes, and the many other involuntary body 

 reactions. 



Ear bones 



Semicircular 

 canals 



Cochlea 



Auditory 

 nerve 



Eustachian tube 



Fig. 22.17. Diagram of the human ear. 



The sense organs are also a part of the nervous system since they 

 pick up stimulations and transmit impulses on the sensory nerves to the 

 central nervous system. It is by means of these sense organs that a 

 frog or a man keeps in contact with the outside world. We will de- 

 scribe these senses primarily from the human aspect and will point out 

 any primary differences in the frog. 



The sense of sight is localized in a pair of well-developed eyes. The 

 vertebrate eye has a structure somewhat like a miniature camera with 

 an iris diaphragm to regulate the amount of light which enters the eye, 

 a lens which focuses the image, and a retina which receives the image in 

 much the same manner as film is exposed. 



The sense of smell is localized in the nasal cavity where there is a 

 small patch of tissue containing olfactory sense organs. These marvel- 



