480 THE NERVOUS SYSTEM. 



4. Abdominal. Stimulating the skin of the abdomen causes 

 a contraction of muscles in this region : when this occurs in the 

 epigastric region it constitutes the epigastric reflex. 



5. Nasal. Stimulation of the nasal mucous membrane causes 

 sneezing. 



6. Conjunctival. Touching the eyeball causes closure of the 

 eyelids. 



Deep Reflexes. These are called also tendon reflexes, but are 

 not true reflexes as are the superficial ones, being caused by direct 

 stimulation of the muscles or their tendons. 



1. Tendo Achillis Reflex. If while the extended leg is sup- 

 ported at the knee a hand is firmly pressed against the ball of the 

 foot, a tap on the tendo Achillis causes the gastrocnemius and 

 soleus to contract and draw the heel up quickly. This may exist 

 or not during health. 



2. Ankle-clonus. The leg being supported, the ball of the foot 

 is suddenly pressed so as to put the muscles of the calf on the 

 stretch, and there results a series of clonic contractions of these 

 muscles which cease when the pressure is removed. This is 

 absent in health. 



3. Patellar Reflex or Knee-jerk. If one thigh is crossed over 

 the other, a tap on the tendon below the patella causes a forward 

 movement of the leg. This is present in health, but may be in- 

 creased or abolished in disease. 



Other Functions of the Cord as a Nerve-center. The power of 

 the spinal cord to respond to afferent impulses independently of 

 the will is of great advantage in preserving the body from injury. 

 The attempt to retain one's equilibrium after slipping on a side- 

 walk, and the raising of the arms in front of the face to ward off 

 an unexpected blow, are both instances of this action. 



Walking r , playing on musical instruments, and similar acts are 

 all performed under the influence of the gray matter of the cord. 

 To start them requires the action of the brain, but when once they 

 are begun their continuance is accomplished by the cord, and the 

 brain can be busy about other things without interfering in the 

 slightest degree with the perfection of their performance. Indeed, 

 any attempt to control them is more apt to hinder than to help 

 them. Thus in coming rapidly down a flight of steps, if the 

 spinal cord is permitted to take charge of the act the descent will 

 be made with ease and safety, but if each step is made as the 

 result of volition, the chances of stumbling or of tripping are very 

 much increased. 



The reflex action of the cord may be diminished by shock to 

 the nervous system ; thus in the frog immediately after decapita- 

 tion the reflex power cannot be excited, but after a short time it 

 manifests itself under the influence of a stimulus. A similar 



