110 THE VERTEBRATE ANIMAL: IRRITABILITY 



rone is one of the nerve cells of the gray matter of that part of the 

 spinal cord; and the effector, a certain muscle lying under the 

 region of stimulated skin. The knee-jerk reflex in man is perhaps 

 even simpler. When the tendon of the muscle that extends the 

 knee is tapped sharply just below the knee cap, the stimulus is 

 received by receptors in the tendon. The impulse is transmitted 

 to the spinal cord over an afferent neurone, and, passing through a 

 synapse in the gray matter, is carried back by way of an efferent 

 neurone to the muscle of the region. This effector, by its reac- 

 tion to the discharge of the impulse, produces the extension of the 

 knee. In both of these examples the impulse is transmitted to an 

 effector which is in the same region as the receptor. The analogy 

 to light reflection is apparent. 



In the great majority of cases of reflex action, we find the effect 

 produced at some part of the body other than that at which the 

 stimulus is received. If the skin of a dog's back be rubbed with 

 a pointed implement, the animal will respond by attempting to 

 scratch the place of stimulation with its hind leg. The receptors 

 in this instance are located at the roots of the hairs of the region 

 of the back that is stroked. The afferent neurone carries the 

 impulse to the spinal cord over the dorsal root of the spinal 

 nerve that supplies the skin region involved. Within the gray 

 matter of the cord, the sensory neurone has a synapse with a neu- 

 rone of which both the cell body and the processes lie entirely inside 

 the spinal cord. Over the processes of such a neurone the nervous 

 impulse passes posteriorly along the spinal cord to the level of exit 

 of the nerves of the hind legs. Here a synapse occurs with an 

 efferent neurone and the impulse leaves the spinal cord over the 

 ventral root of a spinal nerve, passing to muscles (effectors) 

 that produce the scratching motion. 



In this type of reflex there are three neurones concerned (Fig. 

 61 A). The neurone along which the impulse passes within the spinal 

 cord is known as the adjustor neurone. Adjustor neurones are very 

 numerous in the central nervous system and make possible the 

 varied reactions that a single stimulus can produce. For example, 

 when acid is applied to the skin of a frog's back, the first reaction, 

 as has been pointed out, is a contraction of the body-wall muscles 

 in the region stimulated. Very soon, however, this is followed by 

 other reactions which can be easily observed in a frog from which 

 the brain has been removed (Fig. 61 B). Th^ fore leg on the side 



