338 MUSCULAR ACTIVITY AND FATIGUE 



excitement or emotion which is the accompaniment of the 

 exertion. Two factors seem to be responsible for this. 

 First, the greater intensity of the mental processes means 

 a greater outflow of impulses to the medulla — further 

 quickening of the heart, rise of blood-pressure and depth 

 of respiration. Secondly, it is beheved that owing to 

 sympathetic stimulation of the suprarenal glands, adrenalin 

 is discharged into the blood. This has the effect of in- 

 tensifying and prolonging the effect already produced by 

 the sympathetic impulses to the organs themselves. By 

 the quickened heart-beat and visceral vaso-constriction 

 the maximum diversion of blood to the active tissues 

 is established; by the erection of hairs and secretion of 

 sweat there is an increase in the amount of heat lost. The 

 metabohc needs of the active muscles are met by a dis- 

 charge of glucose from the hver. It is stated, also, that 

 adrenahn accelerates the recovery of the muscles from 

 fatigue. In tliis way adrenahn completes the transforma- 

 tion of the resting into the fighting animal. At the end of 

 exercise, in normal individuals the pulse and respiration 

 rapidly subside and should reach their normal rate within 

 five minutes. 



FATIGUE 



Fatigue is distinguished objectively by a diminished 

 functional capacity, and subjectively by a general feehng 

 of lassitude, tiredness referred to the muscles, and desire 

 for sleep. The two problems which we have to consider 

 are the location and the cause of the diminished capacity 

 for work. 



As to the location, this may be in any of the following 

 structures : (1) muscles; (2) nerve endings; (3) peri- 

 pheral nerve-fibres ; (4) spinal nerve-cells ; (5) synapses ; 

 (6) cerebral cells. 



When the excised muscle is stimulated repeatedly its 

 contraction undergoes a progressive alteration — lengthen- 

 ing of the latent period, slowing of the contraction, diminu- 



