306 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



the skin. Upon removing the skin bearing these holes the endolymphatic fossa 

 of the chondrocranium will be found beneath it. In this fossa are two ducts, 

 the endolymphatic ducts, which open on the skin by the two holes just mentioned. 

 These ducts connect the cavity of the internal ear with the surface. Very care- 

 fully shave off with a scalpel the cartilage of the elevation containing the ear, 

 working on the same side as before. There will soon be noticed a canal in the 

 cartilage containing a delicate curved tube. This tube is the anterior vertical 

 semicircular duct. Continue removing the cartilage, without injuring this duct. 

 The muscles posterior to the ear may also be removed. Another tube will soon 

 be uncovered posterior to the first one; this is the posterior vertical semicircular 

 duct. There will next be revealed a thin-walled chamber, the vestibule, 

 from which the semicircular ducts spring. Continue picking away the cartilage 

 in small pieces, leaving the vestibule and ducts in place. Lastly, a third duct, 

 the horizontal semicircular duct, will be exposed; it is ventral and lateral to the 

 others. When the cartilage has been removed as far as possible from these 

 structures, leaving them in place, the parts of the internal ear may be identified. 

 The ear consists of the vestibule and the three semicircular ducts. The vesti- 

 bule is divisible into a larger dorsal portion, the utriculus, from which the ducts 

 spring, and a smaller ventral portion, the sacculus, located in a pit in the carti- 

 lage. The semicircular ducts are slender tubes, curved in a semicircle, and ter- 

 minating in a rounded sac, the ampulla. Two of them are situated in the vertical 

 plane, the third in the horizontal plane. The ampullae of the anterior vertical 

 and horizontal ducts are in contact with and communicate with each other and 

 also with the utriculus. The ampulla of the posterior vertical duct is situated 

 ventral to the horizontal duct and communicates with the utriculus. In each 

 ampulla will be seen a white sensory patch, or crista, to which a nerve is attached. 

 Inside the utriculus a white mass of sand grains or crystalline material, the 

 otolith, is visible. The endolymphatic duct opens from the medial side of the 

 vestibule, but the connection is not easy to find. 



Draw, showing parts of the internal ear. After making the drawing the 

 utriculus may be opened and the otolith examined. The sacculus will also be 

 better seen as a ventral extension of the vestibule. 



The internal ear has two functions, that of hearing and that of equilibration. 

 The vestibule and ducts are filled with a fluid, the endolymph, while the channels 

 in the cartilage are filled with perilymph. Changes in pressure in the endolymph 

 due either to the impinging of sound waves on the head or to changes in the posi- 

 tion of the head appear to be the stimuli which excite the sensory cells in the 

 ampullae and vestibule, producing in the first case the sensation of hearing and 

 in the second case sensations of the animal's position in the water, enabling it to 

 keep in the desired position. According to the experiments of Maxwell, both 

 (or either) the ampullae and the vestibule control equilibration. The capacity 

 to perceive sounds is presumably limited to the vestibule. 



