DISSECTION OF THE SHARK 19 



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 sur- 

 rounding cartilage, leaving the membranous canals in place. 

 Demonstrate the ampullae 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 (cf . the human relations, 

 Herrick, '18, Fig. 85). 



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. In well preserved speci- 

 mens the recessus utriculi with its sense organ (macula utriculi) 

 may be seen lying ventrally of the superior and horizontal 

 ampullae. The ventral part of the common chamber corre- 

 sponds with the human sacculus and in life contains a large 

 ear stone or otolith, which is usually disintegrated 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-ventral- 

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

 macula lagenae, 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 cristae), 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 no w be removed . C ompare the internal ear of the 

 dogfish with that of man (see diagram in Herrick, '18, Fig. 91). 



