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 vibrissze are specially sensi- 
tive regions. The nerves terminate in a kind of wreath 
formation about the base of the vibrissz. 
All of these sense organs are invisible to the naked eye 
except the Pacinian corpuscles. If the mesentery is held 
up and looked throtigh 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 Fic. 109. Pacinian 
: seein ; : CoRPUSCLE FROM 
a microscope magnifying thirty diam- THE MESENTERY. X 
20. 
eters. All the spinal sensory nerve a 
fibers enter the cord by the posterior ae ae 
root (Figs. 93 and 100). alls substance of 
The Olfactory Organ.—The organ ray wg” © SPathe- 
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 
