OF MYRIAPODA AND MACROUROUS ARACHN1DA. 
291 
carry out the chief currents of the blood from the heart itself, the systemic arteries, 
and the aortic branches ; while on the contrary, like the veins of the Vertebrata, those 
which bring 1 back the blood in the principal organs do not always pursue the course 
of the nerves, nor are of the same dense elastic texture. They are usually exceed- 
ingly delicate, membranous and transparent ; and this is more especially the case 
with those which, covered by the peritoneum, bring back the blood round the sides 
of the body to the heart. An exception however must be made to a system of ves- 
sels in the Scorpion, which, although performing the function of veins in collecting 
the blood that has been distributed through the system by the arteries, partake also 
of the character of arteries in their texture as well as in their function of propelling 
the collected blood into the branchiae for the purposes of respiration. This, which 
may be regarded as a Portal System of vessels, is in close connexion with the sub- 
oesophageal distribution of the arteries which pass backwards into the abdomen from 
the vascular collar formed by the aorta around the oesophagus. 
Arterial Vessels of the Abdomen. — The vascular collar which surrounds the oesopha- 
gus analogous to the aortic arches in the Myriapoda, forms the great supra-spinal artery 
(fig. 31. a. *’.), which may be regarded as the aorta descenclens. This great vessel is 
formed by it immediately anterior to the middle bony arch (b) of the cephalothorax, 
and passes backwards above the nervous cord (s), in the median line, beneath the arch, 
to which it is slightly adherent by fibrous tissue. It was this circumstance, probably, 
which led Professor Muller, who observed it in 1828, to regard it as a ligament. It 
is extended backwards (Plate XV. fig. 33. i.i.i.) along the whole nervous cord ( m.m.m ) 
to the terminal ganglion of the fourth segment of the tail, gradually lessening in 
diameter, and giving off on its under surface a single short trunk (k), both ant erior and 
posterior to each ganglion. These short trunks pass downwards between the nervous 
cords (m), and unite with the system of portal vessels which extends beneath them (n). 
The supra-spinal artery does not give off any other branches in its course through the 
abdomen until it arrives at the terminal ganglion ; so that these azygos branches, 
which seem to distribute very minute vessels to the ganglion itself, and to the two 
nervous cords as they pass between them, may be regarded as analogous to those 
which we have seen given off from the supra-spinal artery on the ganglia in the My- 
riapoda ; while their continuation with the vessels of the portal system beneath, with 
■which they pass outwards, may represent those which are also continued outwards 
along the nerves in the same class. When the artery has reached the terminal gan- 
glion in the tail (Plate XV. fig. 38.), it gives a descending branc has usual to unite 
with the subspinal vessel (n) beneath it, and then becomes a little expanded, and 
produces on each side a small branch on the front of the ganglion ( k . h), precisely 
similar to the first of the terminal branches in the Myriapoda. This accompanies the 
lateral pair of nerves outwards to their distribution in the lateral muscles and the 
surrounding structures. The trunk of the artery then passes over the ganglion and 
immediately gives off another pair of branches (h), which seem first to form an 
MDCCCXLI1I. 2 Q 
