CH. XLVII.] REFLEX ACTION IN MAN 671 



reflex irritability is also seen in the disease called lateral sclerosis; 

 here the lateral columns, including the pyramidal tract, become 

 degenerated, and so the path from the brain to the cells of the cord 

 is in great measure destroyed. In these patients the increase of 

 reflex irritability may become a very distressing symptom, slight 

 excitations, like a movement of the bed-clothes, arousing powerful 

 convulsive spasms of the legs. 



Reflex time. In the frog, 'deducting the time taken in the trans- 

 mission of impulses along nerves, the time consumed in the cord 

 (reflex time) varies from 0*008 to 0*015 second; if the reflex crosses 

 to the other side it is one-third longer. It is lessened by heat, and 

 under the influence of a strong stimulus (see also p. 204). 



Reflex Action in Man. 



The reflexes obtainable in man form a most important factor 

 in diagnosis of diseases of the nervous system ; each action is effected 

 through an afferent sensory nerve, a system of nerve-cells in the 

 cord termed the reflex centre, and an efferent motor nerve; the 

 whole constitutes what is called the reflex arc. The absence of 

 certain reflexes may determine the position in the spinal cord, which 

 is the seat of disease. 



Two forms of reflex action must be distinguished : 



1. Superficial reflexes. These are true reflex actions, and are 

 excited by stimulation of the skin. 



2. Deep reflexes or tendon reflexes. This is a most undesirable 

 name, as they are not true reflex actions. 



Superficial Reflexes. These are obtained by a gentle stimula- 

 tion, such as a touch on the skin ; the muscles beneath are usually 

 affected, but muscles at a distance may be affected also. Thus a 

 prick near the knee will cause a reflex flexion of the hip. 



The most important of these reflexes are : 



a. Plantar reflex: withdrawal of the feet when the soles are 

 tickled. 



b. Gluteal reflex : a contraction in the gluteus when the skin over 

 it is stimulated. 



c. Gremasteric reflex : a retraction of the testicle when the skin on 

 the inner side of the thigh is stimulated. 



d. Abdominal reflex : in the muscles of the abdominal wall when 

 the skin over the side of the abdomen is stroked ; the upper part of 

 this reflex is a very definite contraction at the epigastrium, and has 

 been termed the epigastric reflex. 



e. A series of similar reflex actions may be obtained in the muscles 

 of the back, the highest being in the muscles of the scapula. 



f. In the region of the cranial nerves the most important reflexes 



