_Respiration._--The process of breathing is conducted with greater deliberation during the period of sleep. This reduction is one of the most notable of the circumstances that first attract the attention of the spectator who observes a sleeping person. The average number of respirations per minute, in an adult of twenty-five to thirty years of age, is sixteen. Quetelet remarked[3] that during sleep this number was diminished by about one-fourth. The same fact has been recorded by other observers.[4] Mosso has also noted the fact[5] that there is a change in the type of respiration, the movements during sleep become less diaphragmatic and more largely costal. He furthermore observed that during the waking period the act of inspiration consumed 8-12 of the complete respiratory phase, but during sleep it was prolonged till it occupied 10-12 of the same cycle. The interval between the end of expiration and the commencement of inspiration was also obliterated by sleep. Notwithstanding this relative increase of inspiratory motion, the quantity of air that passes through the lungs is considerably reduced by reason of the diminished action of the diaphragm. A corresponding reduction of the gaseous exchanges between the blood and the external air has been determined by the experiments of Pettenkofer and Voit, Boussingault, Lewin, and other equally competent observers.[6] _Circulation._--During sleep the heart beats less frequently than during the waking hours. Though a portion of this delay must be attributed to the recumbent position, sleep does still further retard the movement of the heart. My own observations upon children in bed exhibit a difference of twelve to sixteen beats between the pulsations when awake and asleep. According to Trousseau[7] the average number of pulsations observed in a group of thirty children, varying in age from fifteen days to six months, was 140 when awake and 121 when asleep. In another group of twenty-nine children, between the ages of six months and twenty-one months, the average was 128 when awake and 112 when asleep. The observations of Hohl and Allix[8] indicate that among very young children the difference between the pulse of sleep and the pulse of wakefulness may equal forty beats. According to Guy (_loc. cit._) the pulse is more variable in the morning than during the afternoon or evening.