224 



THE VITAL FUNCTIONS. 



period, however, not only are these tributary ves- 

 sels obliterated, but the dorsal vessel itself becomes 

 impervious, and shrivels into a mere slender liga- 

 ment ; it is then, of course, incapable of contri- 

 buting to the motion of the blood ; and its former 

 function has now entirely ceased. 



The activity of the circulation as indicated by 



fio^ures have been ensfraved, and to whom I am indebted also for 

 the description which follows: — 



The dorsal vessel of this insect is an elongated and gradually 

 tapering vessel, extending from the hinder part of the abdomen, 

 along the back, towards the head ; and furnished with valves, which 



339 



correspond very nearly in their situation to the incisions of the body. 

 During the changes of the insect from the larva to the imago state, 

 it undergoes a slight modification of form. In every state it may 

 be distinguished into two portions, a dorsal and an aortal. The 

 dorsal portion, which is the one in which a pulsation is chiefly ob- 

 servable, is furnished with distinct valves, is attached along the 

 dorsal part of the body by lateral musdes, and has vessels which 

 enter it laterally, pouring into it the circulating fluid, which is re- 

 turning from the sides and inferior portions of the body. In the 

 caterpillar, this portion of the dorsal vessel extends from the twelfth 

 to the anterior part of the fifth segment. It is furnished with eight 

 double valves, which are formed as Mr. Bowerbank has correctly 

 described them in the Ephemera marginata ; namely, the upper 



