SLEEP. 313 



the cerebrum the cross effect is also manifested, both as regards sensation 

 and motion ; for the paths of both have already decussated lower down. 

 Lastly, it is maintained that many of the radiating fibres of the cere- 

 bral hemispheres pass over from one hemisphere to the other through 

 the corpus callosum ; and, in any case, the two hemispheres, as well as 

 all the other parts of the bilateral nervous centres, are closely connected 

 together by commissural structures both gray and white. 



In reference to sensation and motion, the action of a bilateral ner- 

 vous centre is explained by the bilateral structure of the parts with 

 which it is connected through its nerves; but the unity of the mind, 

 that is of the conscious part of our nature, and its various reactions 

 on the body, seeing that the brain is double, have constituted a per- 

 plexing problem to certain physiologists. Ordinarily both hemispheres 

 probably act together, each part of the two being respectively associ- 

 ated by its commissural connections. But it has been shown that one 

 is sufficient for the persistence of all the mental faculties, and of their 

 determining influence on the body ; for considerable portions of one 

 hemisphere have been cut away by the sword or otherwise, and very 

 much larger portions have been altered or destroyed by disease, and 

 yet all the mental faculties have been preserved. These and other 

 considerations have led to the adoption of the opinion that the mind 

 itself has a dual action, and that it is possible that when two concur- 

 rent trains of ideas or thought pass together through the mind the 

 cerebral hemispheres are acting differently or in a dual manner, though 

 ordinarily they act together. (Wigan.) 



SLEEP. 



Sleep consists in a temporary suspension of the functions of the cere- 

 bral portion of the nervous system. It may be defined to be a period- 

 ical rest of the organ of consciousness, as regards the outer world ; so 

 that this is no longer sensible to its ordinary stimuli. Sleep and the 

 waking state have been described as the result of a kind of antago- 

 nism between the organic and the animal life ; the functions of animal 

 life, governed by the mind, enjoy from time to time, freedom of action, 

 whilst at other times, they are repressed by the organic force acting 

 in obedience to a law of creative nature. (Muller.) The cerebral 

 hemispheres, and the sensory ganglia at their base, like all other parts 

 of the body, undergo, in the exercise of their functions, a waste of 

 tissue ; hence they require rest, that new matter may be added to 

 them, to compensate for the waste and disintegration of their sub- 

 stance. During sleep, however, all the functions of vegetative life 

 continue to be performed. The pulsations of the heart, the circula- 

 tion, the movements of respiration, the interchange of gases through 

 the lungs and skin, and the chemical and mechanical phenomena, which 

 accompany digestion, absorption, secretion, and nutrition, pursue their 

 course. The movements of the muscles concerned in these functions 

 are, however, somewhat less frequent than during the waking state; 

 thus the respirations become slower or fewer in number, though deeper ; 

 the beats of the pulse diminish in number. On the other, hand, the 



