TRANSMISSION. 675 



isms are probably secondary or agonal invasions 

 in spite of their rather frequent occurrence. Fur- 

 ther investigations by Castellani disclosed the pres- 

 ence of a trypanosome (T. castellani) in the cere- 

 brospinal fluid of a large percentage of the cases, 

 and a little later Bruce found this organism in all 

 the cases he had examined. This observation has 

 been confirmed so many times that the trypan- 

 osome is now generally considered as the cause of 

 the disease. 



Sleeping sickness is not contagious in the or- Tsetse * 

 dinary sense, and Bruce furnishes very strong Fly * 

 evidence that it is transmitted by the bite of 

 a tsetse-fly (Glossina palpalis). The distribu- 

 tion of the disease corresponds to the habitat of 

 this fly, and Bruce transferred the infection to 

 monkeys by means of flies which had bitten those 

 suffering from sleeping sickness. Gray and Tul- 

 lach have demonstrated the presence of trypano- 

 somes in the alimentary canal of tsetse flies which 

 were allowed to feed on the blood of patients with 

 sleeping sickness. 



A pronounced lethargy or somnolence is the symptoms. 

 most striking clinical feature of the disease. "The 

 appearance of the somnolent condition is preceded, 

 often for a long time, by prodromal signs, which 

 are so characteristic that the patient's neighbors 

 cannot possibly be deceived as to the fate that 

 awaits him. The victim complains of weakness, 

 langour, dejection, disinclination for work, head- 

 aches, particularly localized over the occiput, a 

 sensation of weight in the head and giddiness. 

 His eyelids tend continually to close and he has a 

 tendency to go to rest at unusual hours of the day ; 

 for this purpose he seeks out lonely quiet spots, 



