COORDINATION 



211 



receptors, most of the surface of the human body is provided with 



PAIN, HEAT, and COLD SENSE SPOTS. 



2. Sense of Taste 



In the higher Vertebrates the sense of taste is restricted to the 

 cavity of the mouth, particularly to the tongue, where special 

 receptors known as taste buds are in communication with the 



Supporting 

 cells 



Sense 

 cells 



Nerve fibers 



B 



Fig. 146. — A, Cells of olfactory epithelium from human nose. B, Cells of 

 a taste bud in epithelium of tongue. The sensory cells terminate externally in 

 hair-like processes which are activated by the chemical stimuli that produce 

 odor or taste. 



brain by two of the cranial nerves; but in some Fishes similar 

 organs are scattered quite generally over the surface of the body. 

 (Fig. 146.) 



3. Sense of Smell 



The special sense organs of smell, or olfactory cells, reside in 

 the membrane which lines a pair of invaginations of the anterior 

 end of the head, termed olfactory pouches. The cells are in 

 communication with the brain by the olfactory, or first pair of 

 cranial nerves. The pouches constitute relatively simple sacs 

 in the lower Vertebrates, but in the air-breathing forms, and 

 especially in the Mammals, the walls of the pouches are thrown 

 into folds, ridges, and secondary pouches. This is necessitated 

 by the concentration of the olfactory surface to the air passages 

 of the nose which lead to the lungs. However, the human ol- 

 factory apparatus has fallen somewhat from the complexity which 

 it attains in the lower Mammals, as is attested not only by its 



