576 HANDBOOK OF PHYSIOLOGY. 



belong to the cord of the higher vertebrata: the difference is only in 

 degree. And if this be granted, it may be assumed that, in man and 

 the higher animals, many actions are performed as reflex movements 

 occurring through and by means of the spinal cord, although the latter 

 cannot by itself initiate qr even direct them independently. 



Cutaneous and Muscle Reflexes. In the human subject two kinds of 

 reflex actions dependent upon the spinal cord are usually distinguished, 

 the alterations of which, either in the direction of increase or of diminu- 

 tion, are indications of some abnormality, and are used as a means of 

 diagnosis in nervous and other disorders. They are termed respectively 

 (a.) cutaneous reflexes, and (b.) muscle reflexes, (a.) Cutaneous reflexes 

 are set up by a gentle stimulus applied to the skin. The subjacent 

 muscle or muscles contract in response. Although these cutaneous 

 reflex actions may be demonstrated almost anywhere, yet certain of such 

 actions as being most characteristic are distinguished, e.g., plantar 

 reflex; glutear reflex, i.e., a contraction of the glutens maximus when 

 the skin over it is stimulated; cremaster reflex, retraction of the 

 testicle when the skin of the inside of the thigh is stimulated, and the 

 like. The ocular reflexes, too, are important. They are contraction 

 of the iris on exposure to light, and its dilatation on stimulating the skin 

 of the cervical region. All of these cutaneous reflexes are true reflex 

 actions. They differ in different individuals, and are more easily elicited 

 in the young. Muscle reflexes, or as they are often termed, tendon 

 reflexes, consist of a contraction of a muscle under conditions of more or 

 less tension, when its tendon is sharply tapped. The so-called patellar- 

 tendon-reflex is the most well-known of this variety of reflexes. If one 

 knee be slightly flexed, as by crossing it over the other, so that the 

 quadriceps femoris is extended to a moderate degree, and the patella 

 tendon be tapped with the fingers or the earpiece of a stethoscope, the 

 muscle contracts and the foot is jerked forward. 



Another variety of the same phenomenon is seen if the foot is flexed so 

 as to stretch the calf muscles and the tendo Achillis is tapped; the 

 foot is extended by the contraction of the stretched muscles. It appears, 

 however, that the tendon reflexes are not exactly what their name im- 

 plies. The interval between the tap and the contraction is said to be 

 too short for the production of a true reflex action. It is suggested 

 that the contraction is caused by local stimulation of the muscle, but 

 that this would not occur unless the muscle had been reflexly stimulated 

 previously by the tension applied, and placed in a condition of excessive 

 irritability. It is further probable that the condition on which it 

 depends is a reflex spinal irritability of the muscle or (exaggerated) 

 muscular tone, which is admitted to be a reflex phenomenon or an ex- 

 ample of automatism in the spinal cord. 



