RELATIONS OF INSOMNIA TO TYPES OF SLEEP 215 



a walk or bicycle-ride in those not in " training," certainly 

 belongs to this second type. One may be very tired through 

 some unusual form of exercise and yet quite unable to sleep 

 on account of the discomfort or pain arising from the strained 

 muscles or joints. 



The third factor contributing to the onset of sleep is the 

 presence in the lymph of certain katabolites or fatigue-producing 

 substances. Although these as a group have not yet been 

 isolated, there cannot be any doubt that the products of the 

 daily activity both of muscles and of the nervous system are 

 responsible for some of the objective and all the subjective signs 

 of fatigue. Preyer^ thought that lactic acid was responsible for 

 somnolence, and Weichart^ more recently has carried out an 

 investigation into the chemical nature of fatigue-toxins. The 

 conditions of fatigue and those of toxaemia have much in 

 common. MacDougal,^ in his interesting paper on fatigue at 

 the meeting of the British Association at Dublin in 1908, gives 

 due prominence to the presence of katabolites as a factor in 

 the production of fatigue — the normal precursor of sleep. He 

 writes — " It seems probable that the resistance of the synapses 

 is liable to be temporarily increased not only locally by the 

 transition of the nervous excitation across them, but also 

 generally by the influence of the waste products of metabolism 

 brought to them in the blood — that they are, in short, very 

 subject to chemical influences of many kinds." To this he adds, 

 " There is something to be said for the view that they (synapses) 

 are the seats of the primary and principal influence of various 

 drugs, possibly of alcohol, chloroform, strychnine and others." 

 This is the now widely accepted theory of the chemical origin 

 of bodily fatigue, which manifests itself in general tiredness 

 and sleepiness. " Sleep or general quiescence of the brain," 

 MacDougal continues, " is the most important of the modes in 

 which the organism protects itself against exhaustion." 



Sleep has, then, a chemical factor; the katabolites (die 

 Erdmiidungs-stoffe) have accumulated so much that the resist- 

 ance of the synapses has been raised to such a degree that 

 nerve-impulses can no longer pass over them ; especially is this 



' " Uber die Ursache des Schlafes," 1877, and " Schlaf," Eulenberg's Eficyclo- 

 piidie, Bd. xvii. 



* " Uber Ermiidungstoxin," yl/?i;?r/«. nied. IVoch. No. i, 1904. 

 ' Proc. Brit. Assoc. Dublin, 1908. 



