18 CIRCULATORY SYSTEM. 



tubes by the older anatomists and physiologists, is accordant with known l'ucts in the 

 economy of the lower animals, among which it is common for an organ to perform a double 

 Amotion. 



The food in the alimentary canal of insects is moved along by a peristaltic motion : the 

 canal, therefore, is supplied with a muscular apparatus. Indeed, from the phenomena 

 exhibited, we mighl infer that the apparatus has a construction and parts such as have 

 place in quadrupeds : thus the coat- of the alimentary canal are three, a mucous, a cellu- 

 lar, and a muscular coat ; the first being the internal one, the muscular the outside, and 

 the cellular in the middle. The mucous coat is thin, often transparent, and difficult to 

 detect. The muscular coat is perfectly developed : its fibres are both longitudinal and 

 circular ; and where the constrictions peculiar to the tube exist, there it is more perfectly 

 developed, or iu greater force. These muscles serve partly as sphincters, and also to keep 

 up a continual movement 



CiRci-LArioN. Insect-; are furnished with a circulatory apparatus : it consistsof a dorsal 

 vessel, which, as its name indicate-, is a tube running beneath the dorsal lace of the ex- 

 ternal covering. This is the central organ, and is equivalent to the heart iu the higher 

 orders of animals ; and the circulation is continued from the dorsal vessel, in channels 

 excavated in the tissues. The structure of the dorsal vessel permit- the blood to flow from 

 the posterior to the anterior part of the animal, by means of a series of valves opening 

 forwards. The blood is transmitted to the thorax and head, the wings and legs, and returns 

 through certain channels denominated veins, which open into the dorsal at different points 

 through the valves ; but the larger portion of the returned fluid collects in the posterior 

 chamber, and passes the whole length of the vessel. The vessel has been, and still is, re- 

 garded by some physiologists as analogous to the heart : others, however, consider it as 

 more analogous to the aorta of quadrupeds. 



It does not seem to be a matter of much importance whether we regard the so-called 

 dorsal vessel a heart, an aorta, or an organ compounded of both, the posterior segment 

 taking more especially the form and function of an aortic vessel. Its visible contractions, 

 as seen through the transparent covering of many insects, furnish ostensibly valid reasons 

 for regarding it a heart. 



The fluids which circulate in the vascular system of insects are usually white, carrying 

 along distinct corpuscules ban ing forms like those which are found in the higher and more 

 perfect animals. 



In connection with the foregoing facts, it should also be stated that the circulation in 

 insects is carried on in vessel- having close proximity with those of another kind, nafnely, 

 the air-vessels, or trachea. Tin' two classes are easily distinguished from each other by the 

 structure of the latter ; for in order that air may traverse the insect system, it is necessary 



