468 PHYSIOLOGY CHAP. 



or effective condition of sleep ; and sleep is the phase of organic 

 regeneration which determines awakening (Biehat, Joh. Miiller). 



The idea of " restorative " sleep is very ancient, and is even 

 alluded to in Homer. It is a matter of common observation that 

 fatigue induces the need of sleep ; that sleep supervenes naturally 

 after a certain time of waking or of work, and is usually the more 

 irresistible and profound the longer the period of waking and the 

 harder the work. 



As the functional activity of waking accelerates katabolic 

 processes and gives rise to a correlative amount of waste products, 

 it is a natural corollary to assume that these waste products 

 actually are the cause of sleep, inasmuch as they exert a hypnotic 

 influence on the nerve-centres. 



W. 1'reyer (1875-70) gave the name of ponogenous substances 

 (fatigue products) to the katabolites that accumulate periodically 

 in the blood during waking activity and produce the need of skvp. 

 Whatever their origin and nature, these pouogenous substances 

 are avid of oxygen, which they extract from the blood. Lack of 

 oxygen in the blood produces that state of depression of the cere- 

 bral functions which is the essential phenomenon of sleep in the 

 higher animals. According to Preyer, in fact, oxidation of the 

 cerebral grey matter is indispensable to its activity in the waking 

 state. Pniiger demonstrated this on the frog, and it has been con- 

 firmed by the latest studies on the metabolism of the nervous 

 system by Verworn, Winterstein, and Baglioni (see Vol. III. p. 270). 



The cause of the hypnosis produced by accumulation of the 

 ponogenous substances has been interpreted in various ways by 

 different authors. Rachel (1893) held that physiological sleep 

 was due more to delayed elimination of these supposed poisons 

 than to deficient oxidation of the nerve-centres. Errera (1891) 

 assumed that the ponogenes are analogous to leucomaines, which 

 were shown by Bouchard to have a narcotic action. Lahusen 

 (1897) supposed that the activity of the nerve-centres produces 

 narcotic autotoxin, which is destroyed during sleep. K. Dubois 

 (1894-95) extended his conclusions on the hibernation of marmots 

 to physiological sleep, and held that it consists in a carbonic auto- 

 narcosis, i.e. on accumulated CO, in the blood. All these are 

 variants of Preyer's hypothesis of ponogenous substances, which 

 seems acceptable enough if we confine ourselves to their central 

 nucleus, and leave aside the questions relating to their origin, 

 nature, and mode of action, either upon the nervous system or 

 upon other tissues. 



But even when reduced to its simplest form, the theory of 

 ponogenous products fails to explain a large number of facts con- 

 nected with physiological sleep. 



There is no parallel between the degree of exhaustion and the 

 depth and duration of sleep ; excessive fatigue frequently causes 



