508 A MANUAL OF VETERINARY PHYSIOLOGY 



The Theories of Sleep. — The theories formed to account for 

 sleep may shortly be stated. One is based on the accumulation 

 of acid waste products in the system ; the source of these lies mainly 

 in the muscles. These tissues, as we have already seen (p. 410), 

 are capable of producing sarcolactic acid. Lactic acid injected 

 into the circulation is said to produce fatigue, and it certainly 

 produces unconsciousness. Sleep after severe muscular work 

 is a necessity, but the acid theory is obviously incomplete, or 

 the idler would need but little sleep. The doctrine of the neurone 

 has been evoked to explain sleep, the suggestion being that the 

 synapses in the cells of the cerebral cortex shrink, and in 

 consequence fail to make connection with the incoming fibres 

 from the outside world. Recently the view has been put 

 forward by Salmon* that sleep is presided over by an organ of 

 internal secretion. This, he considers, is furnished by the cells 

 of the cerebral cortex. 



Hibernal Sleep. — The long period of sleep enjoyed by hiber- 

 nating animals appears ta be protective in character, and repre- 

 sents the tiding over of a period when food supply is defective. 

 A study of the processes which attend this remarkable pheno- 

 menon might be expected to throw some light on the cause 

 of ordinary sleep. It can hardly be urged that in this case 

 the cerebral cells are secreting, a substance, which keeps the 

 animal in a state of such profound sleep that evidences of life have 

 frequently to be carefully looked for. The metabolism occurring 

 during hibernation has been referred to at p. 384. Salmon 

 draws attention to the remarkable activity occurring in the 

 hibernal gland. This gland lies beside the thymus, and when 

 stored may reach the whole length of the body. It is highly 

 vascular and charged with a fatty substance, rich in lecithin, 

 which accumulates during the summer and is consumed during 

 the period of sleep, so that at the end of the winter the gland, 

 like the animal, may be a mere shred. This gland contains a 

 colloid substance, and it is significant that a similar material is 

 found in the blood of hibernating animals and in no others. 

 No definite statement can, however, at present be made, as to 

 whether the gland furnishes the sleep-inducing material as well 

 as the food supply. 



Psychical Function. — In attempting to define to what extent 

 the faculty of reasoning exists in animals we are treading on 

 distinctly controversial ground. Probably this question can 

 only be positively answered in the affirmative for two animals — 

 viz., the elephant and the dog. With the horse the moral sense f 



* British Medical Journal, July 8, 191 1. 



•f The use of this term is open to objection in the case of animals, but it 

 appears to the writer that something equivalent to the moral sense does 



