OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
283 
is extended, becoming 1 gradually smaller as it approaches the posterior segments of 
the body. The situation of this interesting structure, relatively to that of the nervous 
cord, the alimentary canal, and the other organs of the body, its connexion by vascular 
arches with the last ventricle of the heart, and the course of the blood which it dis- 
tributes, all strongly remind us of its similarity to the great aorta of Vertebrata, and 
of the analogies which the whole of the vascular structures in these worm-like Arti- 
culata bear to those of Fishes and Amphibia, and the earlier condition of the foetus in 
the higher animals. As it passes backwards along the cord, this spinal artery (Plate 
XIV. fig. 26. a) gives off a pair of branches (b) above the anterior part of each gan- 
glion. These branches pass diagonally backwards and outwards, and on reaching 
the base of the first pair of nerves from the ganglion each gives off a large vessel (c) 
that takes the course of the nerve, and is distributed with it to the muscular and other 
structures. It then proceeds to the origin of the second, or principal nerve from the 
ganglion, and gives off a still larger branch ( d ). This, like the first, pursues the 
course of the nerve which is given to the great muscles of the legs, and is distributed 
with it to those structures. After this it turns a little backwards and gives off its 
third branch (e), which accompanies the corresponding nerve from the ganglion, and 
is given with it to the sides of the body. It is then greatly reduced in size, and gives 
off its fourth and smallest branch (/), which accompanies the fourth and smallest nerve, 
that is given exclusively to the tracheal vessels and parts concerned in respiration. 
From the origin of this branch to the fourth nerve, a very minute branch (g), the 
continuation of the trunk, proceeds diagonally backwards and inwards to the median 
line above the ganglion, to become united again with the great spinal vessel (h), and 
thus forms a channel back again into the great trunk for any residual small quantity 
of blood that is not passed into the other branches : the two principal vessels on 
each side thus form a complete vascular circle above each ganglion. Besides these 
principal branches each lateral trunk gives off on its inner side a small branch to 
supply the ganglion itself (i), and other minute vessels are given from the sides of the 
great artery to the cord. This is the distribution of the vessels from the spinal artery 
on each ganglion until it has reached the last segment of the body, in which it is 
scarcely more than one-third of its diameter at its origin in the third segment. Im- 
mediately above the last ganglion the artery itself is divided into two principal 
branches, with only a very minute median one between them. These branches take 
the course of the terminal nerves of the cord, and are distributed with them to the 
last pair of legs and the surrounding structures. These are the distributions of the 
arterial structures in the Myriapoda ; some further idea of the extensiveness of which 
may be formed from the circumstance that the structure of the heart itself is exten- 
sively supplied with nutrient branches (fig. 18. z). A small artery passes along the 
median line of the heart, on its dorsal surface, included between the median suspen- 
sory muscles. This vessel gives off a pair of branches in each segment, about the 
middle of each chamber, and these are ramified on its upper surface. Some of the 
2 p 
MDCCCXLIII. 
