204 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



ORGANS OF TASTE 



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

 buds. These differ morphologically from the lateral-Une organs in 

 having each sensory cell extend the depth of the bud, ending at the 

 basal membrane, while the majority of the supporting cells are on the 

 outer side of the bud. Each sense cell bears a short, bristle-like 

 percipient structure on its free end, while the basal end is embraced 

 by the fibrillae of the nerve. According to some accounts of the de- 

 velopment the taste buds are derived from the entoderm, the only 

 case apparently established for the origin of sense organs except from 

 the ectoderm, but this has not been demonstrated beyond a doubt. 

 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 circumvallate papillae, on the soft palate and on the epiglot- 

 tis. 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 posttrematic 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' formed by fibres from the 

 seventh and sometimes of the tenth nerves. 



OLFACTORY ORGANS 



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

 logically, being what might be called the chemical senses, the organs 

 concerned differ considerably in structure and relations. The ol- 

 factory epithelium is always restricted to one or two patches at the 

 anterior end of the head and differs from the taste buds in histological 

 structure. Both sensory and supporting cells of the olfactory organs 

 are variously constituted. The supporting cells are the stouter, some 

 being ciliated, some muciparous at their free ends. The sense cells 

 (fig. 198, C) are thread-like or rod-like, being greatly expanded 

 around the spherical nucleus, while the basal end of each contracts to 

 a nerve fibre which extends back to the olfactory tract (p. 183), where 



