CIRCULATION OF THE BLOOD. 413 



embryo, the blood originally flows without vessels ; and only after the 

 stream has acquired some degree of regularity, do the vessels form 

 themselves around it. The same appears to be the case in the motion 

 of the juices in the lower animals. In these also the circulating fluid 

 forms for itself a passage through the parenchyma of the body ; it 

 grooves as it were a course for itself, in which it afterwards constantly 

 continues. This course is in insects attracted especially to the large 

 tracheae, because the vital air, that substance to which all blood must 

 attain, is transmitted through them. Were the thick tunics of a 

 vessel to be formed around it, the deposition of oxygen could not so 

 easily take place ; and indeed in insects it would have greater diffi- 

 culties to contend with than in any other class, for in them the 

 tracheae, even to their extreme ends, retain their hard spiral filament, 

 whereas in the vesicles and cells of the lungs and gills it disappears, 

 whence the oxygen can more easily pass through the delicate mem- 

 brane of the respiratory apparatus, and ai*rive at the likewise delicate 

 tunic of the blood-vessels ; but in insects it is more strongly retained, 

 and would be even more so if the blood-vessel also had a thick 

 membrane. It thence appears to me that the deficiency of blood- 

 vessels is necessary to the undisturbed corporeal functions of insects ; 

 their organisation merely required a central organ whereby the motion 

 of the juices is promoted, and by means of which it is regulated 

 and guided ; and this organ is their dorsal vessel. The course 

 through it being originally traced, and the first impulse to the mo- 

 tion of the blood being given by the spontaneous motion of the dorsal 

 vessel, the free stream of blood necessarily follows this direction until 

 it again returns within the sphere of the activity of this organ, and 

 is then again forcibly attracted to it, and, as before, involuntarily 

 driven into its preceding course. 



