DISSECTION OF THE SHARK 19 



lateral border of the head. Locate again the pores of the endo- 

 lymphatic ducts and avoid injury to these ducts. Now remove 

 the cartilaginous roof of the brain of the right side, beginning at 

 the opening which you will find already made by a cut between 

 the eyes and shaving off the cartilage in thin slices. 



9. The internal ear lies embedded within the cartilage behind 

 the eyes close to the medulla oblongata under the pores of the 

 endolymphatic ducts (see Fig. 1). The semicircular canals of 

 the ear can be seen through the translucent cartilage. The three 

 canals should be exposed by dissecting away the surrounding 

 cartilage, leaving the membranous canals in place. Demon- 

 strate the ampulla of the semicircular canals and the connections 

 of each canal with the utriculosaccular chamber. Note the 

 planes in which the canals lie in relation both to each other and 

 to the long axis of the body. 



Draw the dissection of the membranous labyrinth at this 

 stage of the dissection without removing it from the head. The 

 shape of the utriculosaccular chamber can best be seen while it 

 is still in place, for its delicate walls collapse when removed. 

 The semicircular canals open freely into the dorsal part of the 

 common utriculosaccular chamber, which accordingly corre- 

 sponds with the human utriculus. The ventral part of the com- 

 mon chamber corresponds with the human sacculus and in life 

 contains a large ear stone or otolith, which is usually disinte- 

 grated in the formalized specimens. In the wall of the sacculus 

 there is a large sensory area, the macula sacculi. There is no 

 cochlea; but from the sacculus a small pouch extends caudo- 

 ventralward. This is the lagena, which contains a sensory area, 

 the macula lagense, and represents the rudiment from which the 

 cochlea of higher animals has been developed. Through the 

 translucent walls of the membranous labyrinth the whitish 

 sensory areas can be seen (maculae and cristas), and by a little 

 further dissection the branches of the VIII nerve to all of these 

 sensory areas can be demonstrated. The entire membranous 

 labyrinth may now be removed. Compare the internal ear of 

 the dogfish with that of man (see diagram in Herrick, '15, Fig. 

 91). 



Very complete descriptions and figures of the ears of different 

 species of fishes, includinge th dogfish, are given by Retzius 



