Integrative Systems 195 



shaped and encapsuled. Pacini's corpuscles are located not only in 

 the deeper layers of the skin, but also in the mesenteries, tendons, 

 and periosteum. Those present in tendons give rise to sensations such 

 as those which serve to indicate the position of a limb. Those in the 

 deeper parts of the body probably give rise to the painful sensations 

 associated with disease. The spherical or club-shaped Golgi-Mazzoni 

 corpuscles, in which the nerve-termination is coiled, occur in the 

 dermis and in the conjunctiva of the eye (see p. 198). Similar 

 corpuscles, but not "cutaneous," occur in the peritoneum. 



The lateral-line organs of fishes and aquatic amphibians (de- 

 scribed in connection with the account of the cranial nerves on p. 164) 

 are cutaneous sense-organs, but differ from the others in being some- 

 what localized and in their linear arrangement. 



ORGANS OF TASTE AND SMELL 



The skin of aquatic invertebrates is usually sensitive to substances 

 dissolved in the external water. Vertebrate skin, except in some fishes, 

 seems to be quite devoid of chemical sensibility. But the oral cavity 

 and the nasal cavities are lined by a membrane whose superficial layer, 

 the oral and nasal epithelium, is continuous with the external epi- 

 dermis and, like the epidermis, is of ectodermal origin. The oral and 

 nasal cavities are developed by formation of ingrowing ectodermal 

 pouches and are permanently freely accessible to the external medium. 

 In all vertebrates, more or less extensive areas of the epithelia of these 

 cavities possess organs of chemical sense. The oral and nasal chemical 

 organs are similar in that they are stimulated by substances dissolved 

 in water or in a watery mucus. They are similar also in that they are 

 stimulated by substances of the same general nature and are espe- 

 cially important in the detection and selection of food. Aside from 

 their difference in location, they differ structurally in these two re- 

 spects: the nasal receptors are sense-cells scattered singly throughout 

 the sensitive area of the epithelium, while the oral receptors are com- 

 pact clusters of sense-cells; the nasal receptor produces its own con- 

 ductor-fiber which extends back into the brain, while the oral receptors 

 transmit impulses to terminal fibrils of nerves whose cell-bodies are in 

 ganglions of cranial nerves (Fig. 167). The sense mediated by the nasal 

 receptors is "smell"; that by the oral receptors is "taste." The 

 similarity of these two senses is such that, at least in man, it is difficult 

 to separate the sensations which they produce. Most of the flavors of 

 our food we unthinkingly credit to taste although, in fact, these sen- 

 sations of flavors are derived from the nasal or olfactory sense-organs. 

 The oral gustatory organs give us only sensations of sweet, sour, 

 bitter, and salty substances. 



