222 LECTUKE XVII. 



lulus and Scolopendra, and indicate its great advance beyond the 

 condition of the dorsal artery in the Anellides. 



In the perfect Hexapod Insect, the heart has the appearance of a 

 series of slightly conical segments, partially sheathed one upon the 

 other: lateral apertures exist at the sides of the intus-susceptions, 

 where, in fact, valvular folds of the inner tunic do project into 

 the interior of the heart, and partially divide its cavity into so 

 many separate chambers. The vs'hole of this part of the heart is 

 included in a saccular venous sinus, from which the blood passes into 

 the interior of the heart, and, by the disposition of the valves, it is 

 at once prevented from returning into the sinus, or passing in any 

 other direction in the heart than towards the head, or into the next 

 chamber in advance of that by which the fluid was admitted. The 

 number of venous orifices varies in different insects : — in most species 

 there are eight pairs of apertures ; in the stag-beetle there are six 

 pairs ; in the humble-bee five pairs; in the phasma there is, according 

 to Midler, only a single pair at the posterior chamber of the heart, 

 by which, in fact, in all Insects, the chief currents of the blood 

 appear to enter the organ. As far as the head the blood is propelled 

 from the heart along a tubular aorta of the usual foi-m ; but the 

 branches from this would appear soon to lose themselves in the 

 generally diffused sinuses. In the Myriapoda, however, the blood is 

 continued in a vessel along the dorsal aspect of the ventral nervous 

 chord ; but the traces of the true tubular vascular system are scanty 

 and obscure, 



Cuvier, misled by the anomalous diffused condition of the venous 

 system, supposed that there was no circulation of the blood in Insects ; 

 yet the dorsal vessel was too conspicuous a structure to be overlooked. 

 Such, however, was the authority of the great anatomist, that the 

 nature of the heart began to be doubted, and the strangest functions 

 to be attributed to it. Hunter was better prepared to appreciate 

 the true state of the circulating system in insects, by his discovery 

 of the approximatively diffused and irregular structure of the veins 

 in the Crustacea, and has described in his Work on the Blood * 

 all the leading characters of the circulation in Insects as it is recog- 

 nised by Comparative Physiologists of the present day. He says, 

 that, " As the lungs of the flying Insect are placed through the whole 

 body, the heart is more diffused, extending through the whole length 

 of the animal ; " that " where the veins near the heart are large, there 

 is no auricle, as in the lobster and generally in insects ; " that " in 

 the winged Insects, which have but one heart, as, also, but one cir- 



* Qiiarto, 1794. p. 220. et seq. 



