STKUCTTJKE OF THE DOESAL VESSEL. 325 



activity, are likewise intimately related to the completeness of their 

 respiration; so that the vital energies of the muscular system are 

 developed to the utmost, endowing them with that vigorous flight and 

 strength of limb which we have already seen them to possess. It must 

 likewise become apparent that, as the blood is freely exposed to the in- 

 fluence of oxygen in every portion of the insect to which the air-tubes 

 reach, one great necessity for the existence of a circulatory apparatus is 

 entirely done away with, and, as we have observed before, all those parts 

 of the vascular system required in other animals for the propulsion of the 

 vitiated blood through pulmonary or branchial organs are no longer 

 requisite ; so that, by dispensing with the complicated structures usually 

 provided for this purpose, the body is considerably lightened. The cir- 

 culation of the nutritive fluids is, in fact, limited to their free diffusion 

 amongst all the internal viscera, and is effected in the following manner : 

 If we examine the back of a silkworm, or of any transparent larva, a 

 long pulsating tube is seen running beneath the skin of the back, from 

 one end of the body to the other. Its contractions may readily be watched : 

 they are found to begin at the posterior extremity, and are gradually 

 continued forwards ; so that the vessel presents a continual undulatory 

 movement, by which the fluid contained in its interior is pushed from the 

 tail towards the head. This dorsal vessel, which may be so well observed 

 in the thin-skinned larva, exists likewise in the perfect insect, although, 

 from the opacity of the integument, its movement is no longer apparent 

 except by the vivisection of the animal. 



(850.) This dorsal vessel, or heart as we shall call it for the sake of 

 brevity, is organized in a very singular manner ; for, instead of being a 

 closed viscus, it communicates most freely, through several wide lateral 

 apertures, with the cavity of the abdomen, and from thence derives the 

 blood with which it is filled. The dorsal vessel is widest in the abdo- 

 minal region ; but is continued, nevertheless, through the thorax into 

 the head, where it terminates as a simple or furcate tube that is, not 

 closed, but open at the extremity. 



(851.) The structure of this remarkable heart has been fully in- 

 vestigated by Straus-Durckheim*, and is extremely curious : it con- 

 sists, in the Cockchafer, of eight distinct compartments, separated from 

 each other by as many valves formed by productions from the lining 

 membrane, and so disposed that the blood passes freely from the hinder 

 chambers into those which are placed more anteriorly, but is prevented 

 from returning in the opposite direction. 



(852.) Each compartment of the dorsal vessel communicates by two 

 wide slits, likewise guarded by valves, with the cavity of the belly ; so 

 that fluids derived from thence will readily pass into the different 

 chambers, but cannot again escape through the same channel. The 

 arrangement of these valves will, however, be best understood by refer- 



* Op.dt. 



