184 ESSENTIALS OF ZOOLOGY 



membranes lining the mouth, nose, vagina, and urethra. The senses 

 of pain and temperature are probably accommodated by unmodified 

 free nerve endings. Sensations of pain may be produced in almost 

 every organ of the animal's body. Meissners tactile corpuscles are 

 located in the dermis generally over the body, but more particularly 

 on the soles of the feet, at the bases of the vibrissae, at the tip of the 

 snout, and tip of the tongue. In human beings the sense of touch is 

 twice as discriminating on the tongue as it is on the finger tip ; almost 

 thirty times as great as that of the back of the hand; and sixty-five 

 times as great as that of the middle of the back. Pacinian corpuscles 

 are organs of pressure sense located in the deeper dermis, mesenteries, 

 certain visceral organs, and at the joints. 



Sense of Smell. — The organ of the sense of smell lies in that part 

 of the mucous membrane lining the posterior part of the nasal 

 cavity. The portion of the mucous membrane containing the sen- 

 sory olfactory cells is known as the Schneiderian membrane and is 

 grayish in color in a fresh specimen. The remainder, the respira- 

 tory membrane, is red in color. The olfactory nerves conduct the 

 olfactory impulses to the brain. This sense is a chemical sense, and 

 stimulation is produced by volatile substances which go into solution 

 in mucus on the membrane. 



Sense of Taste (Gustatory Sense). — The mucous membrane of the 

 tongue contains most of the taste buds of the manmial. Fewer taste 

 buds are found on the soft palate and epiglottis. Some animals have 

 taste buds on the lips and snout, and in the catfish the feelerlike bar- 

 bels around the mouth, and even on the tail, possess taste buds. This 

 sense too is a chemical sense and materials must be in solution be- 

 fore they stimulate the gustatory cells. Many taste sensations are 

 dependent on olfactory cooperation. Taste sensations are grouped 

 into four classes : hitter , sweet, salty, and sour. In the human being 

 taste buds receiving sweet sensations are located in the tip of the 

 tongue, bitter at the base, salty along the sides, and sour (acid) 

 rather generally in all of these areas. The mid-dorsal portion of the 

 tongue is not very sensitive. 



Sense of Sight. — The eye is the special organ of sight, and within 

 it the retina contains the sensory cells. Each eye is held in an orbit 

 which is located in the skull. The conjunctiva is a thin membrane 

 which lines the eyelids and passes over the front of the eye between 

 the lids. This latter portion of it is transparent. Tear (lacrimal) 

 glands, which are located in the orbit under the base of the upper 



