864 



CENTRAL NERVOUS SYSTEM 



may be lacking. The presence of these cutaneous senses is obvious to 

 any one, and the sensations which they arouse form the basis of many 

 volitional acts. The presence of receptors in the deeper parts of the 

 body, on the other hand, may be recognized only by careful introspec- 

 tion, as when some pathological condition produces deep pain, or de- 

 ranges the unconscious motor responses which depend on the receptors 

 of deep sensibility. That afferent nerve impulses may arise from the 

 muscles, connective tissue, tendons and joints is shown by a number of 

 considerations. We are accurately aware of the position of our limbs 

 and of any change in their position, whether made actively or passively. 

 This knowledge does not depend on the cutaneous sensibility because the 

 sense of position is not impaired when the cutaneous sensibility is 

 destroyed by cutting the sensory nerves to the skin. When this is done 

 sensations of pain and pressure may still be felt if the limb be pressed 

 upon forcibly. Painful sensations which are referred directly to the 

 muscles may arise during a muscular cramp or in the soreness which fol- 

 lows the severe use of muscle groups which are unaccustomed to such 

 activity. The fibers conducting afferent impulses lie in the trunks of 

 the motor nerves, and when these are damaged muscular sensibility is 

 lost. On cutting the dorsal root of a spinal nerve, it is found that a 

 number of fibers in the motor nerve trunks to the muscles undergo 

 degeneration. The receptors in the muscles, connective tissue, tendons, 

 and joints may give rise to sensations of pain and touch and give in 

 addition information which we shall see is invaluable in coordinating 

 the movements of the body (page 914). 



The receptors of the internal organs of the trunk may give rise to 

 sensations of pain, referred either to the inside of the body or to some 

 region of the skin. That the sensibility of these organs is of a low or- 

 der is attested by the fact that in healthy life we are rarely aware of 

 their presence in spite of the almost continuous activity of many of these 

 organs. The study of the distribution of the internal receptors for pain 

 has been made by Lennander 14 during operations performed under local 

 anesthesia. It has been found that the connective tissue about the ten- 

 dons, the synovial membranes, periosteum, and perichondrium, are all 

 very sensitive to pain. The parietal peritoneum is very sensitive to 

 pain, especially from traction. On the anterior abdominal wall, at least, 

 it is not endowed with end organs of pressure, heat, or cold. The mesen- 

 teries are free from pain when cut, but are sensitive to traction. When 

 free from connective tissue periosteum or perichondrium, the bone sub- 

 stance and marrow, cartilage, and arteries and veins are insensitive to 

 cutting, except those bony structures such as the maxilla and teeth which 

 are traversed by sensory nerves. The muscles are insensitive to cutting, 

 although excruciating pain may arise in them as the result of a cramp. 



