PHYSI0IXX3ICAL PROPERTIES OP Till. SIMPLE REFLEX IRC B13 



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

 and the muscles perform the contractions accessary in produce flexion 

 of the liml). Although the refractory period is unaffected by the strength 

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

 nerve reflex arc, Buch as thai caused by changes in blood supply or by 

 narcosis. 



Reflex conduct inn is much less resistant than aerv adnction t<> various 



conditions affecting the nutritive condition of the conducting pathway. 

 For example, deprivation of oxygen causes hut slight interference with 

 the conduction along ;i nerve trunk, hut 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 C 

 of drugs such as chloroform, 0.3 \«v cent of the drug may be required to 

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

 cient to abolish it in a reflex are. 



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

 flex are, we learn many facts concerning the importance of the latter. 

 and we further see that the differences ;ire due very largely to the 

 synaptic connection. 



SUCCESSIVE DEGENERATION 



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

 fjir ))i(tlio<] of successivi 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 oi gment to another. 

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



nmnner: 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, bo 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 W as made across 



tl >rd 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 eellfi 



which the afferent nerve fibers coming From the skin of the Bcratch re- 



tlex area wer innected. A section of the cord, Btained appropriately 



for degenerated fibers, ai this time demonstrated these fibers t. 



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. 



