OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
289 
together, as well as the proper organs of locomotion, the trunks of the arteries still 
preserve their original distinctness, and enable us to identify the organs and parts of 
the head in the Scorpion with corresponding structures that exist under other forms, 
although endowed with similar functions, in the more distinctly developed head of 
the Myriapodes and insects. 
When the aorta (figs. 27. 28. 29. 33.) has descended on the alimentary canal, at a 
short distance behind the brain, it gives off its second great trunk backwards (1.) to 
the posterior pair of legs. The first trunk (i) having passed downwards, unites with 
its fellow of the opposite side below the oesophagus to form the supra-spinal artery, 
the great systemic continuation of the aorta backwards, into the abdomen, the distri- 
bution of which I shall presently describe. The third trunk (2.) is given to the 
penultimate pair of legs, and the fourth (2*.), which is smaller than the others, is 
specially distributed to the middle portion of the thorax. Opposite to this pair the 
aorta is considerably widened, and is separated into three great divisions (f, g, h). 
The middle one of these {/), which also is divided into three branches, still preserves 
its original distinctness, and constitutes the cephalic artery and arterial vessels of 
the head above the oesophagus, while the two great lateral divisions of the aorta are 
given at the sides and below the oesophagus to the two remaining pairs of legs (3, 4.), 
and the great prehensile claws (5.), the analogues of the foot-jaw’s in Scolopendra, 
and the mandibles of lulus and of insects. In this mode of origin and distribution, 
we obtain a clue to the identity of the special organs of the head in the Scorpion with 
those of the more perfect Articulata, and also with those of the mouth in the Chilo- 
poda, the foot-jaws, which many naturalists have regarded as not forming part of the 
organs of manducation. The size of these prehensile organs in the Scorpion requires 
that an immense volume of blood should be supplied to them in the least difficult 
manner, and this is effected by the trunk of the great artery being extended into 
them at an angle very little diverging from the direct line of action of the heart and 
aorta, so that the blood is necessarily propelled onwards through their extensive 
ramifications of vessels, with as little resistance as it is passed into those of the head. 
Before the great lateral artery enters the prehensile organs, which it always does on 
the inner side of the great nervous trunk, it gives off a large branch that ascends in 
front of the brain (6.), and forms a transverse anastomosis with the middle cephalic 
artery (14.), and also with its fellow on the opposite side ; and then passing upwards, 
it is distributed to the large salivary glands (fig. 39.), to the muscles on the upper 
surface of the thorax, to the diaphragm, and to the anterior parts of the dorsal sur- 
face of the abdomen*. This then we may regard as the proper distribution of the 
supracesophageal blood-vessels of the thorax. 
Cerebral Arteries . — In the distribution of the cerebral arteries we find that the 
cephalic artery (PlateXIV. fig. 28./), which we saw in the Myriapoda divided into three 
* These two vessels are often united into a single trunk on the front of the brain, and are then divided 
again and distributed as described. 
