454 



LECTURE XIX. 



In the spiders the heart {^fig. 167, a) extends, as in flying insects, 

 along nearly the whole of the abdomen, but is wider than in the 

 scorpion or in insects. It is fusiform, with thick walls, composed 

 chiefly of transverse muscular fibres, slightly decussating on the 

 inner surface : a narrow strip of longitudi- 

 nal fibres extends along the middle of the 

 dorsal surface. 



The blood is returned to the heart either 

 by from four to six pulmonic vessels (i, h) on le: 

 each side, or by the great sinus that sur- 

 rounds it like a pericardium, or by both ways. 

 It is propelled forwards by the contraction 

 of the muscular walls, which action can fre- 

 quently be discerned through the thin inte- 

 gument of the smaller spiders. M. Duges *, 

 who succeeded in throwing a solution of car- 

 mine into the heart from behind forwards, 

 affirms that it flowed readily by the lateral 

 pulmonic vessels (6, h) to the base of the pul- 

 monary lamellaB, and that these productions 

 of the breathing sacs were coloured rose-red 

 by the injection of their capillaries. Hence he 

 concluded that those vessels were pulmonic 

 arteries. M. Blanchardf, who has equally 

 succeeded in injecting them, deems them to 

 be veins, and the sole channels by which the blood returns to the heart. 

 The disposition of the venous pericardial sinus inclines me, how- 

 ever, to believe that the heart serves the purposes of both pul- 

 monic and systemic circulations, as Hunter discovered to be its 

 function in the flying insect. An artery is continued from both 

 extremities of the heart ; the anterior aorta (c) gives ofi" two trans- 

 verse branches {d\ the posterior vessel soon divides into the genital 

 arteries (e). It sends off" an inferior branch to the intestine, and, 

 having penetrated the thorax, it gives two branches, which divide to 

 supply the gastric cjeca. It then divides into two branches, which 

 run forward nearly parallel to the brain and ej'es ; supply the chelicers 

 and the poison-organ of that part; and bend down to become inti- 

 mately blended with the suboesophageal nervous mass. The arteries 

 have few ramifications, are usually short, and soon lose themselves in 

 the diffused venous sinuses. The venous apertures are bivalvular, 

 as in the heart of the scorpion. The blood contains colourless round 

 corpuscles, which have been seen to circulate in the limbs of young 



* CCLXXIII. t CCLXXVIII. 



Heart of Spider. Pholcus. 



