966 CENTRAL NERVOUS SYSTEM 



urine and feces. About the same time the knee-jerk becomes elicitable. 

 This reflex is obtained by tapping the tendon which connects the patella 

 with the tibia, the response being a smart contraction of the extensor 

 muscles of the knee joint. The flexion reflex also begins to reappear. 

 This is elicited by applying a pinprick or other hurtful stimulus to 

 the skin of a lower extremity, and when fully developed consists in a 

 flexion of the knee and hip joints. The evident object of this move- 

 ment is that the stimulated parts may be removed from the source of 

 stimulation, and it is plain that all stimuli that produce the flexion 

 reflex are such as would cause in the intact animal a sensation of pain. 

 Such stimuli are thus classified as nocuous, and the reflex is styled a 

 nociceptive reflex. Accompanying flexion of the stimulated limb the op- 

 posite or contralateiral limb usually undergoes a definite extension, 

 called the crossed extension reflex. That the nociceptive reflexes should 

 be among the first to return after spinal transection is of considerable 

 interest as indicating their importance in the protection of the animal 

 from injury. They are the essential reflexes of defense, and it is con- 

 siderably later in the recovery of the animal before reflexes dependent 

 upon stimulation of other tactile receptors begin to show themselves. 

 The most important of this latter group of more special reflex move- 

 ments 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 

 backward 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, neither will the application of heat, nor 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 



