SECTION XV 

 THE FUNCTION OF THE SPINAL CORD 



CHAPTER XLIX 



THE SPINAL CORD AS A REFLEX CENTER ITS POWER OF 



AUTOMATICITY 



Localization of the Spinal Reflex Centers. While it is true that 

 the segmentalism so clearly betrayed by the lower forms, is also in 

 evidence in the mammals, it must be admitted that it has lost much 

 of its original conspicuousness on account of the development of the 

 long conducting system and of those central complexes of neurons 

 which give rise to psychic and other singular activities. In endeavor- 

 ing to compare the conditions found in a typical segmental or reflex 

 animal, such as the crayfish, with those existing in man, it may be 

 advantageous to begin this discussion with a general survey of the 

 structural and functional arrangement of the spinal paths in the 

 intermediate groups of animals formed by the reptilia and amphibia. 

 We have previously noted that a stimulus applied to the foot of a 

 decerebrated frog, eventually induces muscular contractions which 

 lead to a retraction of the leg from the seat of the stimulation (Ttirck's 

 method). If the spinal cord is now thoroughly destroyed with the 

 aid of a thin wire, it will be found that subsequent to this time the 

 stimulus remains absolutely ineffective. This result proves that 

 the destruction of the spinal cord has produced in this case a break 

 in the circuit of this particular reflex. Secondly, as the receptor and 

 effector, as well as the afferent and efferent paths, have not been in- 

 terfered with in this instance, the aforesaid procedure must have led 

 to a destruction of the center necessary for this reflex. Thirdly, 

 inasmuch as all other reflexes occurring in the realm of the spinal 

 cord, have also been abolished, the deduction may justly be made 

 that this part of the nervous system contains the centers for a large 

 number of reflex circuits and may, therefore, be regarded as an impor- 

 tant seat of reflex action. 



We have thus established one of the two most important functions 

 of the spinal cord, the other being its power of conduction by means 

 of which the actions of peripheral parts are correlated with those of the 

 cerebrum and allied structures. It is probably not necessary to remind 

 the student of the fact that the destruction of this part of the nervous 

 system does not abolish all reflex action. Only those reflexes are 



594 



