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 utriculus, from which 

 these two ducts spring. Continue picking away the cartilage in small pieces, leav- 

 ing all parts in place. A third duct, the horizontal semicircular duct, lying below 

 and lateral to the others, will next be exposed. When the cartilage has been re- 

 moved as far as possible, the parts of the internal ear may be identified. The 

 central chamber to which the ducts are attached consists of a smaller dorso- 

 anterior portion, the utriculus, from whose dorsal tip the anterior vertical and 

 horizontal ducts spring; and a larger ventro-posterior part, the sacculus, from 

 which the posterior vertical duct takes origin. The sacculus fits into a rounded 

 depression in the cartilage. The semicircular ducts are slender tubes, curved in a 

 semicircle and each terminating in a rounded sac, the ampulla. The ampullae 

 of the anterior vertical and horizontal ducts are in contact with and communicate 

 with each other and also with the anterior extension of the ventral part of the 

 utriculus. This extension is called the recessus utriculi. Both ends of the posterior 

 vertical canal join the sacculus. In each ampulla will be seen a white sensory 

 patch or crista, to which a branch of the auditory nerve is attached. Larger sen- 

 sory patches, called maculae, also occur in the recessus utriculi and in the sacculus. 

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

 is visible. The movements of these grains may be concerned in equilibration. 

 The endolymphatic duct opens from the medial side of the sacculus but the con- 

 nection is difficult to find. 



Draw, showing parts of the internal ear. After the drawing has been made, the 

 sacculus may be opened and the otolith examined. 



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

 The whole structure is 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 position 

 of the head appear to be the stimuli which excite the sensory cells of the cristae 

 and maculae, producing in the first case the sensation of hearing and in the 

 second 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 

 cristae and the maculae control equilibration. The capacity to perceive sounds is 

 presumably limited to the cristae. 



