REFLEXES IN THE SPINAL ANIMAL AND SPINAL SHOCK 805 



That the nociceptive reflexes should be among the first to return after 

 spinal transection is of considerable interest as indicating their im- 

 portance in the protection of the animal from injury. They are the 

 essential reflexes of defense, and it is considerably later in the recovery 

 of the animal before reflexes dependent upon stimulation of other tac- 

 tile receptors begin to show themselves. The most important of this 

 latter group of more special reflex movements include the so-called 

 scratch reflex and the extensor thrust. The scratch reflex, as its name 

 implies, is the scratching movement of flexion and extension of the hind 

 limb at a rate of about four contractions per second that occurs when 

 a mechanical stimulus is applied to the flank and shoulder area of the 

 animal. For example, if we gently draw a pencil or the fingers back- 

 ward and forward among the hairs on this region of the spinal animal, 

 the corresponding hind limb will be brought up so that the claws are 

 approximately at the place stimulated, and the limb thus directed will 

 undergo a series of flexions and extensions, designed evidently for the 

 purpose of scratching the area of skin that has been stimulated. If the 

 stimulus is a weak one, only the initial stages of the movement may 

 occur, such as the preliminary flexion of the leg. As we have already 

 stated, the receptive stimulus calling forth this reflex is very specific 

 in nature. A pinprick or rough friction of the reflex area will not produce, 

 it, nor will the application of heat or of a single electric shock. The 

 most adequate stimulus is one simulating as nearly as possible the con- 

 dition which would be produced by the movement on the flank of the 

 animal of some insect. This more or less complicated scratch reflex can 

 of course also be elicited in animals whose spinal cord has not been cut, 

 but we can not predict in such cases whether the reflex will occur. The 

 brain may inhibit the reflex arc and prevent the movement. In a spinal 

 animal, however, the reflex always occurs provided an adequate stimulus 

 is applied. The great importance of the scratch reflex in the study of 

 the physiology of the spinal cord rests in the fact that a large stretch 

 of cord is involved in the reflex path. The afferent impulses must enter 

 at a much higher level than the efferent impulses leave, and between 

 these two points there must exist a long intraspinal neuron (see page 

 813). This permits us to study many conditions influencing reflex action 

 which otherwise in a reflex located in one segment only it would be im- 

 possible to investigate. 4a 



The extensor thrust is elicited by applying pressure to the pad of the 

 paw or the sole of the foot- It consists of a quick extension movement 

 of the corresponding limb usually with a flexion of the opposite limb. 



After complete recovery from shock, the paralyzed parts of the body 

 are capable of performing even more complex movements than those al- 



