238 THE VITAL FUNCTIONS. 



to the superior segments of the abdomen. At i 

 is seen a layer of the same fibres, which are 

 partly ligamentous and partly muscular, passing 

 underneath the dorsal vessel, and forming, in 

 conjunction with the layer that passes above it, 

 a sheath, which embraces and fixes that vessel 

 in its place : these inferior layers have been 

 removed from the other parts of the vessel, to 

 allow the upper layers to be seen, as is the case 

 at L. Fig. 335 gives a side view of the anterior 

 extremity of the same vessel, showing the curve 

 (a) which it describes as it bends downwards in 

 its course towards the head. 



The function performed by the dorsal vessel, 

 which, judging from the universal presence of 

 this organ in insects, must be one of great im- 

 portance in their economy, was long a profound 

 mystery. Its analogy in structure and position 

 to the dorsal vessels of the Arachnida and the 

 Annelida, where it evidently communicates with 

 channels of circulation, and exhibits movements 

 of pulsation resembling those of insects, was 

 a strong argument in favour of the opinion that 

 it is the prime mover of a similar kind of circu- 

 lation ; but then, again, this hypothesis ap- 

 peared to be overturned by the fact that no 

 vessels of any kind could be seen extending 

 from it in any direction ; nor could any channels 

 for the transmission of a circulating fluid be 

 detected in any part of the body. Those organs, 



