12 THE INSECT WORLD. 



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

 adopted. 



But in 1827 a German naturalist named Cams discovered that 

 there were real currents of blood circulating throughout the body, 

 and returning to their point of departure. The observations of Cams 

 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 " Lecons sur la Physiologic et 1'Anatomie 

 compare'e," 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 con- 

 tractile ; 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 membranous 

 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 constrictions 

 into chambers. These chambers have exits placed in pairs, and 

 membranous 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 membranous 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 into 

 the heart from the pericardic chamber, the empty space surrounding 

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



The anterior or aortic portion of the dorsal vessels shows neither 

 fan-shaped lateral expansions, nor orifices, and consists of a single 

 membranous tube. The whole of the blood set in motion by the 

 contractions of the cardial portion of the dorsal vessel runs into the 

 cavity of the head, and circulates afterwards in irregular channels 



