82 VETERINARY PHYSIOLOGY 



I. THE SPINAL ARC. 



Reflex Action. 



When the cerebrum and cerebelkim are destroyed, the 

 frog is under the influence of the spinal arcs alone, and it 

 may be used for studying the action of these arcs uncom- 

 plicated by the disturbing effects of the cerebral or cerebellar 

 arcs. 



For the study of the action of the spinal arcs in higher 

 animals, Sherrington has used a dog with the spinal cord 

 cut below the level of the phrenic nerves, which supply the 

 diaphragm — the great muscle of respiration, so that the dog 

 can continue to breathe. 



In such an animal it is possible to record the movements 

 of a limb as a whole by attaching it to a reducing lever, 

 i.e. a lever in which the power is applied to the long limb, 

 so that the extent of movement is reduced on the record. 



The action of any muscle or muscles may be recorded by 

 cutting their tendons and attaching them to the lever. 



In the horse, as a result of injury or disease, the lower part 

 of the cord may be cut off from the higher arcs and its 

 uncomplicated action may be studied. 



Beginning with the frog with the whole brain destroyed, 

 it is found that if one of the toes is pinched the leg is drawn 

 up. This is one of the simplest examples of reflex action. 



Reflex Action may be defined as the response of effectors 

 to the stimulation of receptors ivithout volition being involved. 



Such reflex actions are definite and purposeful. This 

 may be demonstrated by placing a small bit of blotting- 

 paper dipped in acetic acid on the thigh of the frog, when 

 a series of movements will be made, manifestly with the 

 object of removing the paper {Practical Physiology). 

 These movements involve the most perfectly co-ordinated 

 action or a large series of muscles, some contracting, some 

 relaxing, the contraction and relaxation alternating in an 

 orderly manner between the groups of muscles. 



The mechanism involved consists of the receptors stimu- 

 lated, the ingoing nerves to the spinal cord, the great 



