290 THE DOG-FISH. 



and two smaller masses at the sides of the head, between the 

 nose and the orbit. From these masses the tubes diverge 

 to their external openings on the dorsal and ventral surfaces 

 of the head. The tubes are filled with a transparent gelatinous 

 matter, which can be squeezed out from their openings. 



The function of the two canal systems is not known with 

 certainty, but they are generally believed to be sense organs 

 of some kind. The nerves supplying them in the head are 

 the ophthalmic, buccal, and external mandibular branches of 

 the seventh nerve. 



Squeeze the head of a fresh specimen, so as to make the 

 gelatinous matter exude, and so render the openings visible. 



Remove the shin from the dorsal and ventral surfaces of 

 the head, an operation much facilitated by dipping the head 

 into hot water for a few moments ; identify the masses of 

 ampullce and the nerves in connection with them, and trace 

 the tubes outwards and backwards to their openings. Some of 

 the tubes which run along the dorsal surface of the skull, between 

 the cranium and the orbit, are some inches in length. 



Cut out a small piece of the mass of ampullce : put it on a 

 slide in a drop of water : separate the parts with needles : cover, 

 and examine under the microscope. Note the ampullce, their 

 internal divisions, the nerves supplying them, and the tubes. 



