330 



PHYSIOLOGY 



CHAP. 



4L(3) 

 5L(4) 

 6L(5) 



7L(is) 



IS(2S) 



on either side is not impaired. Spinal transectioii and transection 

 at the junction of diencephalon or mesencephalon increase the 

 briskness of the jerk, and after ablation of the Eolandic cortex, on 

 one side, the contralateral ' knee-jerk usually becomes more brisk. 

 Jendrassik noticed that a voluntary movement of the arm at the 

 time the knee-jerk is being elicited augments it ; and that if the 

 jerk is very feeble it may be reinforced by making the patient 

 interclench his fingers and pull them apart strongly (Jendrassik's 

 grip). This is probably due to the fact that con- 

 traction of the arm-muscles relaxes the muscles of 

 the leg, and thus cuts out the tone by which 

 the patellar reflex is inhibited. According 

 to Bowditch and Warren, the effect is most 

 v \\ marked when the patellar tap is delivered 

 x '' < -\. 0'2-0'6 sec. after the voluntary move- 



\ ment of the arm. 



; v v^ X. In close association with the 



; tonic action of the spinal centres is 



N 'v *^.._ the trophic action which they 



exert upon other centres and 

 upon the peripheral tissues. 

 We have already reviewed 

 the arguments which 

 underlie the Wal- 

 lerian doctrine that 

 the body or nu- 

 cleated portion 

 of the neurone 

 represents the 

 trophic centre of 

 all its processes. 

 We know further 

 that in the 

 central nervous 

 system the normal trophic influence is exerted in the same 

 direction as the physiological conduction of excitation ; it is the 

 sensory neurones that control the nutrition of the motor neurones, 

 and not the reverse. On interrupting the relations of inter- 

 dependent groups of nerve-cells, there is arrest of development 

 (agenesis) if the growth of parts is still incomplete, secondary 

 atrophy if development is already perfect. After section of the 

 sensory roots not only do their central ends degenerate, but trophic 

 changes may be seen in the corresponding motor root cells 

 (Warrington, 1897). 



In this connection we must confine ourselves to the group of 

 well-known * phenomena which show that the spinal nerves and 

 their centres, as well as the centres of the brain, are to some extent 



Fio. 191. Diagram to show nervous 

 mechanism of knee-jerk. (Sherring- 

 ton.) 4L-1S, 4th-7th lumbar and 

 1st sacral roots of Macacus ; the 

 corresponding roots in man are 

 numbered in brackets (the 7th 

 lumbar pair in monkey corresponds 

 to 1st sacral in man) ; cr.n., crural 

 nerve ; sc.n., sciatic nerve afferent 

 paths indicated by dotted lines, 

 efferent by broken lines ; m.ext, ex- 

 tensor muscle ; m.JJex, flexor muscle. 



