PHYSIOLOGICAL PROPERTIES OF THE SIMPLE REFLEX ARC 813 



which the weight of the body is temporarily removed from the ground, 

 and the muscles perform the contractions necessary to produce flexion 

 of the limb. Although the refractory period is unaffected by the strength 

 of the stimulus it is very dependent upon the internal condition of the 

 nerve reflex arc, such as that caused by changes in blood supply or by 

 narcosis. 



Reflex conduction is much less resistant than nerve conduction to various 

 conditions affecting the nutritive condition of the conducting pathway. 

 For example, deprivation of oxygen causes but slight interference with 

 the conduction along a nerve trunk, but very soon abolishes the spinal 

 reflexes. Even in the frog, reflex movements entirely disappear in thirty 

 to forty-five minutes after the centers have been rendered completely 

 anemic, and in mammals they disappear in a few minutes. In the case 

 of drugs such as chloroform, 0.3 per cent of the drug may l>e required to 

 abolish conduction in a nerve, whereas a much lower percentage is suffi- 

 cient to abolish it in a reflex arc. 



From the above differences in conduction in a nerve trunk and a re- 

 flex arc, we learn many facts concerning the importance of the latter, 

 and we further see that the differences are due very largely to the 

 synaptic connection. 



SUCCESSIVE DEGENERATION 



Before concluding the subject, it may be of interest to consider briefly 

 the method of successive degeneration, by which Sherrington succeeded 

 in demonstrating the exact tracts in the white matter of the spinal cord 

 along which the intraspinal neurons travel from one segment to another. 

 This was worked out in the case of the scratch reflex in the following 

 manner: The spinal cord was first of all cut in the upper thoracic region, 

 so that degeneration occurred in all the descending tracts below the 

 level of the section. In about a year's time these degenerated tracts had 

 entirely disappeared, and the debris of the degenerated fibers had been 

 replaced by cicatricial tissue, so that a section of the cord revealed noth- 

 ing but healthy nervous tissue with cicatrices where the degenerated 

 tracts had existed. When at this stage a second cut was made across 

 the cord a little lower than the first one, further degeneration occurred 

 involving those fibers whose centers were located between the two cuts 

 that is, the fibers coming from the intraspinal neurons, with the cells of 

 which the afferent nerve fibers coming from the skin of the scratch re- 

 flex area were connected. A section of the cord, stained appropriately 

 for degenerated fibers, at this time demonstrated these fibers to exist 

 in the lateral column of white matter, those that travel a short distance 

 i. e., between neighboring segments being near the gray matter, and ' 

 those traveling greater distances, towards the outside. 



