178 THE VITAL FUNCTIONS. 



The discovery of the circulation in insects, and of its va- 

 rying energy at different periods of growth, has elucidated 

 many obscure points in the physiology of this important 



ial. The dorsal portion, which is the ojie in which a pulsation is chiefly ob- 

 servable, is furnished with distinct valves, is attached along the dorsal part of 

 the body by lateral muscles, and has vessels which enter it laterally, pouring 

 into it the circulating fluid, which is returning from the sides and inferior 

 portions of tlie body. In the caterpillar, this portion of the dorsal vessel ex- 

 tends from the twelfth to the anterior part of the fifth segment. It is fur- 

 nished with eight double valves, which are formed, as Mr. Bowerbank has 

 correctly described them in the Ephemera marginala — namely, the upper 

 valve " by a reflecting inwards and upwards of the inner coat, or coats of the 

 artery," (by which he means the dorsal vessel) "and the under one by a con- 

 traction or projection of the like parts of a portion of the artery beneath, so 

 as to come within the grasp of the lower part of the valve above it." The 

 whole vessel is made up of three coats, the two innermost of which, the 

 lining, or serous, and the muscular, or principal portion of the vessel, consti- 

 tute the reflected portions, or valves; while the third, or outermost coat, 

 which is exceedingly thin and delicate, is continued over the vessel nearly in 

 a straight line, and does not appear at all to follow the reflexions of the other 

 two. In the caterpillar, this portion of the vessel has eight pairs of small 

 suspensory muscles, seen along the upper side of Fig. 339, which arise from 

 the middle of the upper surface of each valve, and are continued back to be 

 attached over the middle of the next valve: they seem to have considerable 

 influence over the contractions of the valves. The Aortal, or anterior por- 

 tion of the vessel, extends from the hinder part of the fourth segment to its 

 termination and division into vessels, to be distributed to the head, which di- 

 vision takes place after it has passed the oesophagus, and at a point immedi- 

 ately beneath the supra-cesophageal ganglion, or brain of the insect. This 

 portion of the vessel is much narrower than the dorsal, has no distinct 

 valves or muscles; nor do any vessels enter it laterally; but it is very delicate 

 and transparent, and gradually diminishes in size from its commencement to 

 its anterior termination. Its course, in the caterpillar, is immediately beneath 

 the integument, along the fourth and third segments, till it arrives at the 

 hinder parts of the second segment; when it gradually descends upon the 

 oesophagus, and, immediately behind the cerebral ganglion, gives off a pair 

 of exceedingly minute vessels. It then passes beneath the ganglion, and, in 

 the front part of the head, is divided into several branches, as noticed by Mr. 

 Newport in the anatomical description he has given of the herves of this spe- 

 cies of Sphinx: (l^hil. Trans. 1832, p. 385.) These branches are best ob- 

 served in the chrysaUs (Fig. 339:) in all the stages they maybe divided into 

 three sets; the first is given off immediately after the vessel has passed be- 

 neath tlic ganglion; and consists of two lateral tnanks, the united capacity of 



