714 A MANUAL OF PHYSIOLOGY 



from lesions of the central system, and denominated 

 'early' or Mate' according as it comes on within a few days 

 or a few weeks after the occurrence of the lesion, is also 

 probably in part a reflex phenomenon, although possessing 

 some of the characters of a tonic contraction due to auto- 

 matic discharge from the spinal centres. For in such cases 

 myotatic irritability is increased ; the knee-jerk is exag- 

 gerated ; a finger-jerk may be elicited by tapping the wrist, 

 an arm-jerk by striking the skin over the insertion of the 

 biceps or triceps, ankle-clonus by flexing the foot (Gowers). 

 Now, myotatic irritability depends on reflex muscular tone 

 (p. 708). 



It is probable that the tone of such visceral muscles as 

 the sphincters of the anus and bladder has also a reflex 

 element, and possible that the same is true of the tone of 

 the smooth muscular fibres of the bloodvessels on which 

 the maintenance of the mean blood -pressure so largely 

 depends. And it may be that if all afferent impulses could 

 be cut off from the vaso-motor centre, as by section of the 

 whole of the posterior spinal roots and other centripetal 

 paths to the medulla, general dilatation of the arterioles 

 would take place, and the blood - pressure be greatly 

 diminished. But, as has been already more than once 

 pointed out, there exist peripheral mechanisms which, after 

 a time, make good to some extent the loss of tone caused 

 by destruction of the spinal centres (p. 694). 



Trophic Tone. The degenerative changes that occur in 

 muscles, nerves, and other tissues when their connection with 

 the central nervous system is interrupted have been already 

 referred to (p. 605). It is possible to explain these changes 

 in some cases without the assumption that tonic impulses 

 are constantly passing out from the brain and cord to 

 control the nutrition of the peripheral organs ; and we have 

 seen that there is no real evidence of the existence of 

 specific trophic fibres. But the degeneration of muscles 

 after section of their motor nerves is difficult to understand 

 except on the hypothesis that impulses from the cells of 

 the anterior horn influence their nutrition. The only ques- 

 tion is whether these are the impulses to which muscular 



