292 
MR. NEWPORT ON THE NERVOUS AND CIRCULATORY SYSTEMS 
union (k) at the roots of the nerves with the subspinal vessel beneath, and then pass 
outwards to the sides of the segments. Immediately after this a second median 
branch is united behind the ganglion with the subspinal vessel, and the main trunk 
passes on fora short distance between the terminal nerves of the ganglion, and is then 
divided into two branches which take the course of the terminal nerves. Each of 
these divisions of the artery gives off a branch which is distributed in part forwards 
(1.) to the under surface of the colon, and part backwards to the same structure (2.). 
The two terminal divisions of the artery (3.) then pass backwards with the terminal 
nerves and anastomose by a short transverse trunk (4.), and each again divides into 
two branches immediately before the nerves also are divided, at the posterior part of 
the fourth segment. They then accompany the nerves to their ultimate distribution, 
and are always divided immediately before any division takes place in the nerves. 
This is invariably the manner in which the arterial vessels accompany, and are divided 
with, the nerves, and proceed with them to their ultimate distribution. 
Having traced the distribution of the arterial vessels from the anterior extremity 
of the heart, it remains now to follow those of the posterior, which afford some curious 
peculiarities. The last two chambers of the heart which are situated in the seventh 
segment of the abdomen, are greatly reduced in size, and constitute the origin of the 
caudal artery, and seem to be the means by which part of the current of blood is 
directed backwards to the tail. Each of these chambers receives its venous trunks 
(figs. 27. b. 33. t.) in a direction more transversely backwards than the other chambers, 
so that the influx of the received blood is directed backwards. The vessels which 
convey it ascend between the muscles and the peritoneum, as well as around the sides 
of the segments. The visceral arteries from these chambers (p) are also altered in their 
direction. Instead of passing laterally and forwards, they give off only a small trunk 
forwards and to the sides of the segments, while their principal trunks are directed 
backwards. Those from the seventh chamber proceed as far as the posterior part of 
the first caudal segment, at the sides of the colon, and are specially distributed to the 
terminal alimentary cseca, while the eighth chamber (p, g) sends off only a single artery 
on its under surface. This is divided into three branches, two of which pass, one on 
each side, to the terminal cseca, while the third proceeds in the middle line, beneath 
the caudal artery, as far as the middle of the second segment, where it is divided into 
branches which are given to the muscular structure of the colon. When the caudal 
artery has entered the first segment of the tail it gives off on each side a plexus of 
four vessels, two of which are distributed forwards and laterally; the third back- 
wards, to the great flexor muscles on the dorso-lateral surface of the segment ; and 
the fourth (r), passing round the sides, between the peritoneum that covers the mus- 
cular structures and the colon, meets with the nerves that are passing upwards from 
the ganglion of the cord, and pursuing their course downwards unites beneath the 
ganglion (s) with the caudal portion of the subspinal vessel (n) that is passing inwards 
to the abdomen. At the commencement of each segment of the tail a pair of vessels 
