382 



LECTURE XVII. 



152 



extending into other parts of the body and its appendages, resembling 

 the general interspaces of the cellular tissue, and constituting the 

 venous system, through which the blood moves in a definite and 

 regular course to the heart. This organ (yfig. 160, s) is an elongated 

 muscular and valvular tube, situated along the middle of the back, 

 and usually called the dorsal vessel : it is largest in the abdomen, and 

 so diminishes anteriorly, that its continuation in the thorax may, in 

 most insects, be regarded as the aorta. 

 It is retained in its position by flat- 

 tened triangular bands of muscular 

 fibres. This character, its distinct 

 transverse linear muscular fasciculi, 

 its slight constriction at regular in- 

 tervals, and the peculiar valvular 

 loops at these constrictions, charac- 

 terize the long and slender vasiform 

 dorsal heart in the lulus and Scolo- 

 pendra, and indicate a corresponding 

 advance with the rest of the articulate 

 structure beyond the condition of the 

 pulsating dorsal sanguiferous tube in 

 the Anellids. 



In the perfect Hexapod Insect, the 

 heart {fig- 152) has the appearance of 

 a series of slightly conical segments 

 (a), partially sheathed one upon the 

 other: lateral apertures (c, h) exist 

 at the sides of the intus-susceptions, 

 where, in fact, valvular folds (c, d) 

 of the inner tunic do project into the 

 interior of the heart, and, with the 

 semilunar valves (c), partially divide 

 its cavity into so many separate cham- 

 bers. The whole of this part of the 

 heart is included in a saccular venous 

 sinus (a, c), 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 ^**"*' '""^' *" '''"s-'^ett'e- 



direction in the heart tlian 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 



