12 VERTEBRATE ZOOLOGY 



includes the paired cranial and spinal nerves by which the brain 

 and the spinal cord are placed in connection with the various organs 

 of the body, and the sympathetic nervous system, whose function is 

 to innervate the important viscera of the body cavity ; and (3) the 

 special sense organs, by which the animal is placed in touch with 

 its environment. We shall study the last-named organs first. 



The special sense organs include the integumental sense organs, 

 the olfactory organs, the eyes, and the ears. 



The Integumental Sense Organs. Embedded in the skin of the 

 fish are minute sense buds, whose exact function is not known, but 

 which undoubtedly enable the animal to receive certain impres- 

 sions from the water surrounding it. The most important of these 

 in the dogfish are the lateral line and the ampullae of Lorenzini. 



The lateral line is a canal in the integument which extends along 

 each side of the body from the head to the hinder end ; it appears 

 on the outer surface as a light-colored ridge. In this canal are 

 groups of sensory cells which are innervated by the tenth cranial 

 nerve. Each of these groups has a minute opening to the outside. 

 The lateral line also extends on to the head, but is here difficult to 

 see in an ordinary dissection. 



The ampullae of Lorenzini are bulb-shaped bodies at the end of 

 long mucous canals, which open upon the head, and are most 

 numerous on the snout. The external openings of these canals 

 appear as dots upon the skin ; the canals are filled with a trans- 

 parent secretion. The ampullae are collected in three principal 

 groups, the largest of which is on the undersurface of the anterior 

 end of the snout, while the other two are on the side of the head, in 

 front of and between the eyes. From these points the canals pro- 

 ceed to the external openings, which are arranged in rows on the 

 head and vary, consequently, a good deal in length, some of them 

 measuring an inch or more. 



Observe the openings of these canals. Skin the anterior portion 

 of the head and study the arrangement of the canals, tracing them 

 to their ends. 



The Olfactory Organs. Two nasal capsules are present, which 

 open to the outside on the ventral surface of the snout, in front of 

 the mouth. Remove the ventral wall of one of these and dissect 



