THE PHYSIOLOGY OF SLEEP. 239 



The respirations become slower and deeper and the costal respiration, 

 (respiration by elevation of the ribs) predominates over the ab- 

 dominal or diaphragmatic respiration as compared with the waking 

 condition. The respiratory movements also show frequently a 

 tendency to become periodic, that is, to increase and decrease 

 regularly in groups after the manner of the Cheyne-Stokes type 

 of breathing. The expiration is frequently shorter and more audi- 

 ble than in the respirations of the waking hours. The eyeballs 

 roll upward and inward and the pupil is constricted. According 

 to Lombard's observations, the knee-kick decreases or disappears 

 entirely during sleep. Some of the constant secretions are dimin- 

 ished in amount, as, for instance, the urine, the tears, and the 

 secretion of the mucous glands in the nasal or pharyngeal mem- 

 brane. One of the familiar signs of a sleepy condition is the dryness 

 of the surface of the eyes, a condition that leads to the rubbing 

 of the eyes. It is sometimes stated that the digestive secretions 

 are diminished during sleep, but the statement does not seem to 

 rest upon satisfactory observations, and may be doubted. The 

 pulse-rate decreases during sleep and there are also certain sig- 

 nificant changes in the distribution of blood in the body owing 

 to a diminished vascular tone in the skin vessels. These latter 

 changes will be referred to more in detail below. The physiological 

 oxidations are also decreased, as shown by the diminished output 

 of carbon dioxid. On the whole, however, the physiological activities 

 of the body go on much as in the waking condition. Those changes 

 in activity that do occur are, in the main, an indirect result of 

 the partial or complete cessation of activity in the brain. One 

 might say that while the cortex of the brain sleeps that is, is 

 inactive most of the other organs of the body may be awake and 

 maintain their normal activity. Another fact of interest is that 

 the entire cortex does not fall asleep at the same instant nor 

 always to the same extent. Ordinarily as sleep sets in the power 

 to make conscious movements is lost first and the auditory sen- 

 sibility last, and on awakening the reverse relation holds. The 

 individual may be conscious of sound sensations before he is 

 sufficiently awake to make voluntary movements. 



The Intensity of Sleep. The intensity of sleep that is, the 

 depth of unconsciousness has been studied by the simple device 

 of ascertaining the intensity of the sensory stimulus necessary to 

 awaken the sleeper. Kohlschiitter * used for this purpose a pendu- 

 lum falling against a sounding plate. At intervals of a half-hour 

 during the period of sleep the auditory stimuli thus produced were 

 increased in intensity until waking was caused. His results are 

 expressed in the curve shown in Fig. 106, in which the intensity 

 * Kohlschiitter, "Zeitschrift f. rationelle Medicin," 1863. 



