188 



GENERAL ZOOLOGY 



lack pain receptors. Thus surgical operations involving cutting 

 through the skull and removing parts of the brain, can be per- 

 formed under local anesthesia without causing pain. In such 

 cases the anesthetic produces insensibility in the scalp but not in 

 the deeper structures. Cutting or cauterizing the intestine causes 

 no pain, though pain is produced in the peritoneum by the disten- 

 tion of intestine by gas or by the excessive contractions of the 

 muscle in the intestinal wall. In general, since pain is a symptom 

 of danger threatening life, the adaptive significance 

 of the presence of pain receptors throughout the skin 

 can be understood. 



Warmth and Cold Receptors. — If the human skin 

 is explored with warm or cold thin metal rods, it is 

 found that some spots respond with a sensation of 

 warmth and others with a sensation of cold. Evi- 

 dence has been produced to show that the receptors 

 for warmth and cold consist of two distinct types of 

 receptors, viz., the corpuscles of Ruffini for warmth 

 and the corpuscles of Krause for cold. Human skin 

 is composed of a great number of very small sensory 

 areas, each of which is especially related to one or 

 more of the sensations of touch, warmth, cold, and 

 pain. Areas concerned with one sensation are 

 everywhere mingled with areas concerned with 

 others. It has been estimated that there exist in 

 the skin of the trunk and limbs 30,000 warmth spots, 

 25,000 cold spots and 500,000 touch spots, with pain 

 spots present everywhere. 

 Proprioceptors. — Corpuscles of Pacini, ovoid laminated bodies 

 with a nervous core, resemble in their general structure the 

 corpuscles of Meissner. They are found near joints and liga- 

 ments, and are thought to be receptors for proprioceptive sensa- 

 tions caused by movements of one bone upon another, such as 

 accompany voluntary muscular movements. Other types of 

 receptors, such as muscle and tendon spindles, composed of 

 spirally coiled nerve endings, are also concerned in proprioceptive 

 sense. Neuromuscular spindles must not be confused with the 

 end plates of motor nerves which terminate in muscle fibers. 

 The latter cause the muscle to contract when the motor nerve, of 

 which they are the terminations, is stimulated. When the 



Fig. 120 — 

 C o r p vis cl e of 

 Meissner, dia- 

 grammatic. 



