INTRODUCTION. 13 



tion, properly so-called, among insects, his opinion was universally 

 adopted. 



But in 1827 a German naturalist named Carus discovered that 

 there were real currents of blood circulating throughout the body, 

 and returning to their point of departure. The observations of 

 Carus were repeated and confirmed by many other naturalists, and 

 we are thus enabled to form a sufficiently exact idea of the manner 

 in which the blood circulates. 



The following summary of the phenomena of circulation among 

 insects is borrowed from "Lemons sur la Physiologic et 1'Ana- 

 tomie comparee " by M. Milne Edwards : 



The tube which passes under the skin of the back of the head, and 

 front part of the body, above the alimentary canal, has been known 

 for a long time as the dorsal vessel. It is composed of two very 

 distinct portions : the anterior, which is tubular and not contractile, 

 and the posterior, which is larger, of more complicated structure, 

 and which contracts and dilates at regular intervals. 



This latter part constitutes, then, more particularly the heart of 

 the insect. Generally it occupies the whole length of the abdomen, 

 and is fixed to the vault of the tegumentary skeleton by membra- 

 neous expansions in such a manner as to leave a free space around 

 it, but shut above and below so as to form a reservoir into which the 

 blood pours before penetrating to the heart. This reservoir is often 

 called the auricle, for it seems to act as an instrument of impulsion, 

 and to drive the blood into the ventricle or heart, properly so 

 called. 



The heart is fusiform, and is divided by numerous strictures into 

 chambers. These chambers have exits placed in pairs, and mem- 

 braneous folds which divide the cavity in the manner of a portcullis. 

 The lips of the orifices, instead of terminating in a clean edge, 

 penetrate into the interior of the heart in the form of the mouth- 

 piece of a flute. The double membraneous folds thus formed on 

 each side of the dorsal vessel are in the shape of a half moon, 

 and separate from each other when this organ dilates ; but the 

 contrary movement taking place, the passage is closed. 



By the aid of this valvular apparatus, the blood can penetrate 

 the heart from the pericardic chamber, the empty space surrounding 

 the heart, but cannot flow back from the heart into that reservoir. 



