RECEPTION, TRANSMISSION, AND DISCHARGE 125 



mouth (Fig. 69), Certain areas of the skin are sensitive to changes 

 in temperature, others to pressure. In addition to these recep- 

 tors, b}^ means of which the organism is made aware of external 

 environmental changes, there are receptors in all the internal organs 

 of the body. The nature of the specialization involved in receptor 

 surfaces is not understood, but their specificity is well known. 

 Sound waves will not start an impulse in the retina. On the 



-a 



Fig. 68. — Diagram of human ear. 



A, the auditory canal leading from the external ear to the tympanic membrane at B; 

 C, cavity of the middle ear containing the "ear-bones" that transmit vibrations from 

 the tympanic membrane to the inner ear; D, the pharynx with which the cavity of the 

 middle ear is connected by the Eustachian tube; E, semicircular canals of the inner ear; 

 F, cochlea; G, auditory nerve. (From Hough and Sedgwick, "Human Mechanism," 

 copyright, 1918, by Ginn and Co., reprinted by permission.) 



other hand, if the optic nerve is stimulated mechanically, the 

 impulse started results in a visual sensation. The person who says 

 he "sees stars" when he gets a " black eye " has scientific founda- 

 tion for his assertion. Receptors normally are sensitive to only 

 one type of stimulus, and if they are stimulated by unusual methods 

 the impulse produces the typical sensation. This latter effect may 

 be due to the specificity of reaction of the brain center to which the 

 impulse is conveyed. 



The reception of a stimulus by a sensory area is followed by the 



