A text-book of physiology for medical students and physicians . the basisof Mossos experiments, quoted above,we assume that the blood-flow in thebrain stands in a reciprocal relationto that in the arm, this curve may be taken to indicate thatbefore and after the onset of sleep the blood-flow through the * Shepard, American Journal of Physiology, 23, 1909 (Troc. Soc). t Howell, loc. cit. THE PHYSIOLOGY OF SLEEP. 261 brain diminishes rapidly to a certain point and that beforeawaking the blood-flow begins to increase again until it reachesnormal proportions. Effect of Sensory Stimul
A text-book of physiology for medical students and physicians . the basisof Mossos experiments, quoted above,we assume that the blood-flow in thebrain stands in a reciprocal relationto that in the arm, this curve may be taken to indicate thatbefore and after the onset of sleep the blood-flow through the * Shepard, American Journal of Physiology, 23, 1909 (Troc. Soc). t Howell, loc. cit. THE PHYSIOLOGY OF SLEEP. 261 brain diminishes rapidly to a certain point and that beforeawaking the blood-flow begins to increase again until it reachesnormal proportions. Effect of Sensory Stimulation.—That sensory stimuli of vari-ous kinds affect a sleeping individual without entirely awakinghim is shown by the movements that may be caused in thisway, and also by the nature of the dreams which may be pro-voked. It is very interesting to find from plethysmographicobservations that all kinds of sensory stimulations from withoutand from within are liable to affect the circulation of the bloodduring sleep. As shown by the plethysmograph, the volume of. Fig. 117.—Sleep: .4. effect of external impression (music box), insufficient to awakensleeper,—a marked diminution in volume of the arm; B, effect of external impression(music box) sufficient to awaken sleeper; a stronger diminution in volume followed bydilatation as the subject again fell asleep. the arm diminishes more or less in proportion to the intensityof the stimulus, and the probable interpretation of this fact isthat the sensory stimulus acts reflexly upon the vasomotorcenter in the medulla and causes through it a contraction ofthe blood-vessels. In the curve shown in Fig. 116 most ofthe irregularities were traceable to causes of this kind,—noisesin the building or street or other sensory stimuli. The samefact is exhibited in a striking way by the curves given in In these experiments the recorder attached to the plethys-mograph to register the changes in volume was of a differentkind (tambour) and
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