44 VERTEBRATE ZOOLOGY 



external opening exists. To dissect it out is a difficult task, but 

 with care and patience it can be done. The auditory capsule, 

 which contains the membranous labyrinth, is an extensive cavity 

 in the cranium, in communication with the brain cavity ; it also 

 contains the peculiar granular, fatty matter which fills the brain 

 cavity. The labyrinth is embedded partly in this fatty matter and 

 partly in cartilage. It is made up of the saclike vestibule, three 

 semicircular canals, and an endolymphatic duct. 



Carefully shave away the bony wall of the skull just back of 

 the orbit, and look first for the semicircular canals. Two of these 

 are vertical in position and rise to the dorsal wall of the skull, 

 the anterior and larger of the two being embedded in the granular, 

 fatty matter, and the posterior one being inclosed in cartilage. 

 Having found these canals and dissected them free, look for the 

 third semicircular canal, which has a horizontal position. Note 

 that one end of each canal is swollen ; this is the ampulla. 



The semicircular canals project from the vestibule. This is a 

 saccular structure which is composed of two parts, the utriculus 

 and the sacculus, the latter being ventral to the former and being 

 nearly filled by a single very large otolith, — a flattened, very hard, 

 calcareous structure, with prominent serrations at its hinder end. 

 At the anterior end of the sacculus is a small pocket called the 

 lagena, containing a minute otolith ; this is the structure which in 

 mammals becomes the cochlea. The endolymphatic duct is a very 

 slender, straight tube, which passes dorsally from the sacculus and 

 ends blind. It is the remains of the embryonic invagination which 

 during the development of the animal resulted in the formation of 

 the membranous labyrinth. 



Another method of dissecting the ear is to split the skull and 

 brain by an incision in the sagittal plane, to remove the brain, and 

 then carefully to scrape away the fatty matter at the intersection 

 of the auditory and the brain cavity until the membranous laby- 

 rinth is exposed. 



The ear in most fishes is not an organ of hearing but of equi- 

 libration, its function being to enable the animal to maintain the 

 proper position in the water. 



Exercise 12. Draw the membranous labyrinth, so far as observed. 



