REFLEX ACTION. 245 



but it is over 60 mm. for the skin of the back of the neck. The 

 temperature receptors are still more definitely located in areas, 

 some being specialized for heat and others for cold. These so- 

 called heat and cold spots are most frequent on the portions of 

 the body that are covered by clothing, for example, the skin of 

 the thorax, than on those that are exposed, for example, the face. 

 They are fairly frequent on the skin of the dorsum of the hand, 

 where their existence can be very easily demonstrated by slowly 

 drawing a pencil gently over the skin. At certain places the 

 point of the pencil feels hot, at others cold, and in others it 

 causes no temperature sensation whatsoever. 



All varieties of receptors are present on the skin of the hand, 

 but in certain diseases of the nerves or spinal cord, one kind of 

 receptor may become inactive, thus causing, when the absent sen- 

 sation is that of pain, the condition called analgesia, which must 

 be distinguished from that of anesthesia, when all sensations are 

 paralyzed. In analgesia a pin prick causes only a sensation of 

 touch. When the nerves of the arm are cut and the cut ends 

 then sutured together so that the nerve fibers regenerate the skin 

 sensations do not all return at the same time. Those of pain and 

 of extreme degrees of heat and cold return in from six to twenty- 

 six weeks, whereas those of touch and the finer degrees of tem- 

 perature do not return until after one or two years. The power 

 of localizing the point of application of the stimulus is also late 

 in returning; thus, if we touch the finger of such a person and 

 ask him to tell us where, he may indicate some spot that is quite 

 a distance away from the one actually touched. Certain drugs, 

 such as cocaine, have the power, when applied locally, of ren- 

 dering all the receptors insensitive. 



THE AFFERENT FIBRE. Another name for this is the sensory 

 nerve, because it carries the sensations received by the receptors 

 up to the nerve center. All afferent fibers enter the spinal cord by 

 the posterior nerve roots, on each of which, it will be remem- 

 bered, is situated a ganglion, the posterior root ganglion. The 

 cells of this ganglion are connected with the afferent fibers by 

 a short branch running at right angles to the latter (Fig. 41). 

 The function of the cells is to main-tain the nutrition of the affer- 



