OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
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upper surface of each chamber a little beyond the outlets of the systemic arteries, and 
prevents the return of the blood from the anterior of the chamber. 1 am not certain 
whether these valves are extended around the whole of the interior of the chamber, 
but believe that they exist chiefly at its upper and lateral surfaces, and are almost 
absent on the under. 
The systemic arteries (figs. 18 and 19. h) given off from each chamber of the heart 
supply nearly the whole of the blood to the viscera and sides of each segment. They 
are each divided into two branches (i) at a short distance from their origin, and pass 
diagonally forwards in the segments, giving off numerous ramifications in their course. 
One of the main branches always passes backwards and outwards, and the other for- 
wards and downwards among the viscera. The latter of these gives branches to the 
alimentary canal and to the ovaries, which are situated between it and the dorsal 
vessel, or heart. The posterior pair of these arteries, which belong to the second 
chamber (20), in the last segment of the body are of small dimensions, and seem to be 
given chiefly to the rectum and the terminations of the organs of generation. Their 
place is in a great measure supplied by the arteries of the third and fourth chambers, 
the largest and most extensively distributed of the whole series. These arteries I have 
examined more minutely than any of the others. Those which belong to the fourth 
chamber (18. h ), in the nineteenth segment, pass almost directly outwards to the 
hepatic vessels (n) and divide immediately into two great trunks (i, k). One of these 
passes (m) backwards and downwards to the anterior of the twentieth segment, first 
giving off a secondary branch that distributes some minute ramifications to the coats 
of the hepatic vessels in its course ; and then, turning inwards, it gives some branches 
to the investing peritoneal coat of the tracheal vessels in the twentieth segment ; and 
then passes downwards ( o ) into the segment and divides into three branches, which 
are given in part to the colon and in part to the pyloric extremity of the stomach. The 
other branch (/), the principal division of this great artery, passes forwards in the 
course of, and very close to the hepatic vessel. Soon after it has separated from the 
posterior branch it gives off some small vessels that go to the great hepatic vessel 
and ramify in its texture ; and others that pass downwards to the interior of the 
segment. It then accompanies the hepatic vessel (n) as far as the middle of the seg- 
ment, where it meets with a large tracheal vessel (p) that is passing inwards to be 
distributed to the organs of reproduction. At that point it gives off another branch, 
which also passes downwards in the segment, and is given to the muscles and fatty 
structures. The main branch then winds round the trachea that passes between it 
and the hepatic vessel, and gives off some small ramifications that pass inwards with 
the trachea to their distribution. The main trunk of the artery (q) then pursues its 
course along the hepatic vessel as far as the seventeenth segment, where it forms some 
anastomoses with the systemic arteries of the fifth chamber. Throughout the whole 
of its course along the hepatic vessels, to which this branch of the artery is chiefly 
distributed, it gives off numerous small vessels that ramify in its coats, and other 
