May 5. 1898] 



NATURE 



mm.; but if the pressure be taken of the same subject 

 lying in bed, and quietly engaged on mental work, it will 

 be found to be no higher. By mental strain or muscular 

 effort, the pressure is, however, immediately raised, and 

 may then reach 130-140 mm. of mercury. It can be 

 seen from considering these facts that the fall of pressure 

 is concomitant with rest, rather than with sleep. As, 

 moreover, it has been determined on strong evidence 

 that the cerebral vessels are not supplied with vasomotor 

 nerves, and that the cerebral circulation passkely follows 

 every change in the arterial pressure, it becomes evident 

 that sleep cannot be occasioned by any active change in 

 the cerebral vessels. This conclusion is borne out by the 

 fact that to produce in the dog a condition of coma like 

 to sleep, it is necessary to reduce, by a very great amount, 

 the cerebral circulation. Thus, both carotids and both 

 \ertebral arteries can be frequentiy tied at one and the 

 same time without either producing coma or any very 

 marked symptoms. The circulation is, in such a case, 

 maintained through other channels, such as branches 

 from the superior intercostal arteries which enter the 

 anterior spinal artery. While total anaemia of the brain 

 instantaneously abolishes consciousness, partial anaemia 

 is found to raise the excitability of the cortex cerebri 

 By estimation of the exchange of gases in the blood 

 which enters and leaves the lorain, it has been shown 

 that the consumption of oxygen and the production of 

 carbonic acid in that organ is not large. Further, it may 

 be noted that the condition of an;esthesia is not in all 

 cases associated with cerebral anicmia. Thus, while 

 during chloroform anaesthesia the arterial pressure 

 markedly falls. Such is not the case during anaesthesia 

 produced by ether or a mixture of nitrous oxide and 

 oxygen. 



The arterial pressure of man is not lowered by the 

 ordinary fatigue of daily hfe. It is only in extreme 

 states of exhaustion that the pressure may be found 

 decreased when the subject is in the standing position. 

 The fall of pressure which does occur during rest or 

 sleep is mainly occasioned by the diminished rate of 

 the heart. The increase in the volume of the limbs is 

 to be ascribed to the cessation of muscular movement, 

 and to the diminution in the amplitude of respiration. 

 The duty of the heart is to deliver the blood to the 

 capilliaries. From the veins the blood is, for the most 

 part, returned to the heart by the compressive action of 

 the muscles, the constant change of posture and by the 

 respiration acting both as a force and suction pump. 

 All of these factors are at their maximum during 

 bodily activity, and at their minimum during rest. On 

 exciting a sleeper by calling his name, or in any way 

 disturbing him, the limbs, it has been recorded, decrease 

 in volume while the brain expands. This is so because 

 the respiration changes in depth, the heart quickens, 

 the muscles alter in tone, as the subject stirs in his 

 sleep in reflex response to external stimuli. Considering 

 all these facts, we must regard the fall of arterial pressure, 

 the depression of the fontanelle, and the turgescence of 

 the vessels of the limbs as phenomena concomitant 

 with bodily rest and warmth, and we have no more right 

 to assign the causation of sleep to cerebral anaemia than 

 to any other alteration in the functions of the body, such 

 as occur during sleep. 



We may well here summarise these other changes in 

 function. 



(i) The respiratory movement becomes shallow and 

 thoracic in type. 



(2) The volume of the air inspired per minute is 

 lessened by one-half to two-thirds. 



(3) The output of carbonic acid is diminished by the 

 same amount. 



(4) The bodily temperature falls. 



(5) The acidity of the cortex of the brain disappears. 



(6) Reflex action persists ; the knee jerk is diminished, 



NO. 1488, VOL. 58] 



pointing to relaxation in tone of the muscles ; con- 

 sciousness is suspended. 



Analysing more closely the conditions of the central 

 nervous system, it becomes evident that, in sleep, con- 

 sciousness alone is in abeyance. The nerves and the 

 special senses continue to transmit impulses, and to pro- 

 duce reflex movements. If a blanket, sufficiently heavy 

 to irnpede respiration, be placed upon the face of a 

 sleeping person, we know that it will be immediately 

 pushed away. More than this, complicated movements 

 can be carried out : the postillion can sleep on horse- 

 back ; the punkah-wallah may work his punkah, and at 

 the same time enjoy a slumber ; a weary mother may 

 sleep, and yet automatically rock her infant's cradle. 

 Turning to the histories of sleep-walkers, we find it 

 recorded that, during sleep, they perform such feats as 

 climbing slanting roofs, or walking across dangerous 

 narrow ledges and bridges. The writer knew of the 

 case of a lad, who, when locked into his room at night 

 to prevent his wandering in his sleep, climbed a partition 

 eight to ten feet in height which separated his sleeping 

 compartment from the next, and this without waking. 



The brain can carry out not only such complicated 

 acts as these, but it has been found to maintain during 

 sleep its normal inhibitory control over the lower reflex 

 centres in the spinal cord. 



Thus, in sleeping dogs, after the spinal cord has been 

 divided in the dorsal region, reflexes can be more easily 

 evoked from the lumbar than from the cervical cord, 

 because the former is freed from the inhibitory control of 

 the brain. 



The strength of stimulus necessary to pass the threshold 

 of consciousness, and to produce an awakening, has 

 been measured in various ways. It has been determined 

 that it takes a louder and louder sound, or a stronger and 

 stronger electric shock to arouse a sleeper during the 

 first two or three hours of slumber ; after that period, 

 the sleep becomes lighter, and the required stimulus 

 need be much less. 



The alternative theories, which have been suggested to 

 account for the onset of sleep, may be classed as chemical 

 and histological. 



In relation to the first, it has been suggested that if con- 

 sciousness be regarded as dependent upon a certain rate 

 of atomic vibration, it is possible that this rate depends 

 on a store of intramolecular oxygen, which, owing to 

 fatigue, may become exhausted ; or it may be supposed 

 that alkaloidal substances may collect as fatigue products 

 within the brain, and choke the activity of that organ. 

 Against this theory may be submitted the facts that 

 monotony of stimulus will produce sleep in an unfiatigued 

 person, that over-fatigue, either mental or bodily, will 

 hinder the onset of sleep, that the cessation of external 

 stimuli by itself produces sleep. As an example of this 

 last, may be quoted the case recorded by Strumpel of a 

 patient who was completely anaesthetic save for one eye 

 and one ear, and who fell asleep when these were closed. 

 Moreover, many men possess the power, by an eflfort of 

 will, of withdrawing from objective or subjective stimuli, 

 and of thus inducing sleep. 



The histological theories of sleep are founded on recent 

 extraordinary advances in the knowledge of the minute 

 anatomy of the central nervous system, a knowledge 

 founded on the Golgi and methylene-blue methods of 

 staining. It is held possible that the dendrites or branch- 

 ing processes of nerve cells are contractile, and that they, 

 by pulling themselves apart, break the association path- 

 ways which are formed by the interlacing or synapses 

 of the dendrites in the brain. Ramon y Cajal, on the 

 other hand, believes that the neuroglia cells are contractile, 

 and may expand so as to interpose their branches as 

 insulating material between the synapses formed by the 

 dendrites of the nerve cells. The difficulty of accepting 

 these theories is that nobody can locate consciousness 



