96 ESSENTIALS OF PHYSIOLOGY. 



pyramidal fibres from each cerebral hemisphere convey impulses to 

 the contralateral side of the body, 



FATIGUE. 



The seat of fatigue in a muscle-nerve preparation is the end-plates. 

 The causation of fatigue, however, in the intact animal is a very complex 

 process, and other factors than the end-plates are concerned, the most 

 important being the central nervous system. Even the fatigue brought 

 about by muscular exercise probably has its origin to a larger extent in 

 the nervous system than in the muscles themselves, the part of the 

 nervous system which becomes fatigued being, in all probability the 

 synapses. The changes in the nerve cells which are brought about by 

 prolonged exercise also indicate that fatigue is partly nervous in origin. 

 It must be remembered also that the sense of fatigue does not correspond 

 exactly with the degree of fatigue as measured by the capacity of the 

 muscles to do work ; and the effect of psychical influences in lessening 

 or abolishing the sense of fatigue is well known. 



SLEEP. 



Every active tissue of the body has alternating periods of activity 

 and rest. Thus the ventricles of the heart have a period of contraction 

 of three-tenths of a second followed by a period of relaxation of five-tenths 

 of a second. In the case of the other muscular tissues and of the 

 glands, the periods are longer and are often irregular, but the same 

 general principle holds good. The active phase of the cells in the 

 cerebral cortex which subserve consciousness coincides with the waking 

 period, while the resting phase is the period of sleep. During sleep 

 consciousness is in abeyance and the activity of all the vital processes 

 is lowered ; respiration is slower, the heart beats more slowly, glandular 

 secretions are reduced in quantity, metabolic changes generally are 

 diminished, and the temperature falls. Histological observations on the 

 nerve cells in sparrows show that certain spindle-shaped, clear bodies 

 (Nissl spindles) disappear from the cells during the activity of the day 

 and are restored during the night's rest. It may be concluded that 

 katabolic changes exceed anabolic during the waking hours, and that 

 the reverse is the case during sleep. 



The cause of sleep has been much discussed, and it is generally 

 agreed that it is associated with a diminished supply of oxygen. It has 

 been shown experimentally that if oxygen is withheld the subject of 

 experiment may become unconscious before he is aware of any un- 

 pleasant symptom. The cells of the cerebral cortex are therefore 

 peculiarly susceptible to a deficiency of oxygen. It has been suggested 



