THE NERVOUS SYSTEM. 



219 



Cutaneous Sense Organs. The cutaneous sense organs 

 are composed of the endings of the sensory nerves in all 

 parts of the skin and the mucous membrane of the mouth, 

 nose, arms, vagina, and urethra. One kind of sense organs, 

 those of pain, are present in every organ of the body. The 

 sense organ of pain is probably an unmodified free nerve- 

 ending. 



While all portions of the skin and perhaps other parts 

 of the body are supplied with organs capable of receiving 

 stimuli giving rise to tactile sensation, the soles of the feet 

 and the skin at the base of the vibrissae are specially sensi- 

 tive regions. The nerves terminate in a kind of wreath 

 formation about the base of the vibrissae. 



All of these sense organs are invisible to the naked eye 

 except the Pacinian corpuscles. If the mesentery is held 

 up and looked through toward the 

 light, the Pacinian corpuscles or sen- 

 sory nerve terminations appear as 

 translucent oval bulbs about two milli- 

 meters long. If a piece of the mesen- 

 tery containing a corpuscle is pinned 

 tense on a piece of cork and then cut 

 out and placed ten minutes in 3% 

 acetic acid, the termination of the nerve 

 within the corpuscle may be seen with 

 a microscope magnifying thirty diam- 

 eters. All the spinal sensory nerve 

 fibers enter the cord by the posterior 

 root (Figs. 93 and 100). 



The Olfactory Organ. The organ 

 of smell lies in that part of the mucous 

 membrane lining the caudal part of the nasal cavity and the 

 basal third of the ethmoturbinal bones (Fig. 18). That 

 part of the mucous membrane containing the olfactory cells 



FIG. 109. PACINIAN 

 CORPUSCLE FROM 

 THE MESENTERY. X 

 20. 



ax, Axis-cylinder ; n, 

 neurilemma ; m, the 

 white substance of 

 Schwann ; e, epithe- 

 lial cell. 



