218 THK VITAL FUNCTIONS. 



divided into a number of chambers by partitions, 

 which act as valves : at the base of each partition 

 there are valvidar openings in the sides, so con- 

 structed as to admit of the passage of blood from 

 the abdomen into the vessel ; but preventing its 

 egress when once admitted. Each portion of the 

 dorsal vessel contracts in succession, from behind 

 forwards, and thus carries onwards the blood it 

 contains into the next chamber, which is dilated for 

 its reception. This alternate action is assisted by 

 an arrangement of transverse ligamentary bands, 

 apparently containing layers of muscular fibres, 

 which embrace the vessel, and attach it to the 

 neighbouring integuments. The blood, impelled by 

 this succession of pulsations, which are frequently, 

 however, irregular or intermittent, issues out from 

 the open orifices of the aorta, or its immediate 

 branches, into the cavity of the head ; whence it 

 returns, in retrograde streams, along the sides of 

 the body. It is afterwards again taken up by the 

 abdominal portion of the dorsal vessel ; and the 

 circuit which it thus performs answers all the 

 purposes of a circulation to these animals. 



An enlarged representation of the dorsal vessel 

 of the Melolontha vnl(>'aris, or common cockchaffer, 

 isolated from its attachments, is given in Fig. 333, 

 showing the series of dilatations (v, v, v) which it 

 usually presents in its course ; and in Fig. 334 the 

 same vessel is exhibited in connexion with the 

 ligamentary and nmscular apparatus which sur- 

 rounds it, seen from the lower side. In the last of 

 these figures, a is the tapering prolongation of the 

 tube, proceeding towards the head of the insect ; 

 v, one of the dilated portions, or ventricles, as they 



