242 



DISCOVERY 



that the only position in which puppies cannot go to 

 sleep is that in which their heads are kept lower than 

 their bodies. The retina in the interior of the eye is 

 a part of the brain ; if the retina be examined with the 

 ophthalmoscope by someone who is familiar with 

 the blood-supply of the eye of a waking person, it will 

 be found in sleep to be distinctly paler. 



The rest of the evidence is more indirect, but to the 

 physiologist equally cogent. The Italian physiologist 

 Mosso contrived to make a man go to sleep balanced 

 accurately on a plank or table ; as the man fell asleep 

 the end of the table where the feet were dipped down 

 through an angle corresponding to about the weight 

 of 260 c.c. of blood. Evidently this is due to a redis- 

 tribution of the blood, there being relatively less at 

 the head end and more at the feet end of the body. 



This redistribution of blood during sleep may be 

 studied in yet other ways. We all know that the 

 skin is flushed in sleep, noticeably so in children and 

 persons with transparent skins — hence the " sleeping 

 beatuy " — but this means that if now the skin holds 

 more blood, the brain will be holding less. 



A Russian physiologist has proved that the pressure 

 in the carotid arteries of dogs asleep is less than 

 during waking. The American physiologist Howell 

 has proved, by a delicate apparatus in which he en- 

 closed the arm of a sleeper, that as he fell asleep the 

 blood in the arm was increased, and therefore had been 

 diminished in the brain. It seems clear, then, that the 

 cutaneous and cerebral blood supplies can vary simul- 

 taneously in opposite directions. 



But the skin is not the only place where the blood 

 which is leaving the brain may be found ; some of it 

 may be accommodated in the internal organs, especially 

 those of digestion. One has only to recall the sleepiness 

 that many people experience after a full meal, for the 

 simple reason that the stomach in active digestion 

 needs a great increase of blood which it must withdraw 

 from the brain. 



In the last analysis it is the fall of blood- pressure in 

 the vessels of the brain which is the vascular factor 

 leading to somnolence, and therefore anything which 

 reduces the pressure tends to sleep. Thus before an 

 attack of sea-sickness blood is leaving the head, as is 

 shown by the pallor of the face ; most of us know that 

 we feel decidedly sleepy before the vomiting occurs. 

 Persons exposed to e.xtreme cold become very sleepy 

 through the enfeeblement of the heart devitalised 

 by the low temperature. Old people, owing to the 

 weakness of their cerebral circulation, often drop off 

 to sleep, especially when tired and in a sitting posture. 

 This was noticeably so during the last few months 

 of the life of Queen Victoria, who would frequently 

 be foimd asleep in the carriage on her afternoon 

 drive. 



Evening, and its Accompaniments 



We may now ask ourselves why it is that as evening 

 comes on the skin and intestinal organs are accom- 

 modating relatively more blood than earlier in the day. 

 The answer is that the small arteries of these systems 

 are losing some of the tone they had earlier in the day. 

 Most people have noticed their collars, rings, etc., 

 feel tighter towards evening. Vessels whose tone is 

 diminished dilate, and therefore hold more blood than 

 before ; in this way the dilated vessels of the skin 

 and viscera accommodate blood which was previously 

 in the brain, and therefore sleep results. 



Finally, it may be asked, " Why do these vessels 

 dilate towards evening ? " The answer is, the nerve 

 centre whose duty it is to keep them partially con- 

 tracted is somewhat fatigued and fails to keep them 

 as fully constricted as it did earlier in the day. The 

 insomnia related to this vascular factor is very familiar. 

 If the heart is beating too rapidly and strongly, it 

 maintains so vigorous a flow of blood through the 

 brain that the cells there are kept in a state of activity 

 which, as we have seen, is incompatible with sleep. 



Anything which spurs on the heart to increased 

 effort is inimical to sleep. Thus the stimulant effects 

 of violent exercise, of certain drugs, of not excessive 

 quantities of alcohol, of blood raised one or two degrees 

 in temperature, are all in the direction of banishing 

 sleep. The well-known hypnotic effect of alcohol 

 in hot water (toddy), or of hot miUc, or of hot milk 

 and alcohol is entirely due to the vascular factor, 

 for these warm foods cause so abundant a dilatation 

 of the vessels of the stomach that blood is withdrawn 

 from the brain and somnolence ensues. The mere 

 fact of going to bed hungry is a source of sleeplessness 

 due to the positive sensory factor, i.e. hunger ; but 

 the partaking of a particularly indigestible meal 

 shortly before bedtime is, by reason of the unusually 

 violent muscular movements aroused in digesting it, 

 a much more serious source of a sleepless night. 



A hot bath, in that it dilates the vessels of the skin 

 and so lowers cerebral pressure, is an excellent hyp- 

 notic ; and the Turkish bath with its subsequent 

 massage is still more soporific. 



What Causes Insomnia ? 



It is extremely probable that the onset of healthy 

 sleep is due to the co-operation of all the four factors 

 which we have discussed. Thus the mind being free 

 from too obtrusive thoughts and sensations, and there 

 being a certain degree of brain fatigue, a fall of cere- 

 bral blood- pressure occurs and the person falls asleep. 



The reversal of any of these four conditions — psychic, 

 sensory, fatigue, or vascular — will involve the corre- 

 sponding insomnia. 



