OLFACTORY ORGANS. 189 



and while some fishes have been proved to hear, others have given 

 negative results. With the terrestrial vertebrates the sound percipient 

 functions of the ear are beyond a doubt, while they still retain their 

 equilibrational use. The sound waves strike the tympanic membrane, 

 are carried across the middle ear by the auditory ossicles, and set the 

 perilymph in motion and thus affect the parts of the membranous 

 labyrinth. 



Organs of Taste. 



The sense of taste is resident in groups of cells known as taste buds. 

 These differ morphologically from the lateral line organs in having 

 each sensory cell extend the depth of the bud, ending at the basal mem- 

 brane, while the majority of the supporting cells are on the outer side of 

 the bud. Each sense cell bears a short, bristle-like percipient struc- 

 ture on its free end, while the basal end is embraced by the fibrillas of the 

 nerve. According to the accounts of the development the taste buds 

 are derived from the entoderm, the only case apparently established 

 for the origin of sense organs except from the ectoderm. In the higher 

 vertebrates the organs are restricted to the cavity of the mouth where 

 (mammals) they occur on the tongue, especially on and near the cir- 

 cumvallate papillae, on the soft palate and on the epiglottis. In the 

 fishes the distribution is much wider, for they are found in the pharynx, 

 on the gills, and in many species on the surface of the body, even upon 

 the tail. The barbels about the mouth of many forms are richly 

 supplied with these organs. 



The taste organs are supplied by different nerves. Apparently 

 those of mammals are supplied by the chorda tympani and the lingual 

 branch of the ninth nerve. In the fishes those of the pharyngeal 

 region are supplied by the post-trematic branches of the glossopharyn- 

 geal and vagus; those of the mouth by the palatine and mandibular 

 branch of the seventh; while those on the head of teleostomes are 

 supplied by the ophthalmic and maxillary branches of the fifth; and 

 those of the trunk by the nerve of Weber (p. 173), formed by fibres 

 from the seventh and sometimes of the tenth nerves. 



Olfactory Organs. 



While the senses of smell and taste are closely associated physiolog- 

 ically, being what might be called the chemical senses, the organs con- 



