288 
MR. NEWPORT ON THE NERVOUS AND CIRCULATORY SYSTEMS 
arteries (w), through which the blood is transmitted to the sides of the segments. 
The reason for this imperfect structure of the valves may perhaps be explained by the 
fact that the blood is distributed from the heart in the Scorpion in opposite directions, 
partly backwards to the tail, but chiefly forwards and outwards to the head and sides, 
as in the Myriapoda, and hence it may be necesary that a reflux of the blood should 
not be entirely prevented, as may be required in those instances in which the whole 
current is in one direction. This also may be the reason for the valves being formed 
more transversely, and less completely than in insects. The structure of the organ is 
exceeding thick, opake, and muscular. It is formed of two layers of fibres, longitu- 
dinal and circular in each layer, the most powerful of which are the latter. On its 
internal surface it is smooth, and lined by an exceedingly delicate membrane, through 
which the strong circular fibres are distinctly marked. It is by means of these that 
its most powerful contractions are effected, the auricular action being chiefly the 
result of the relaxation of these fibres, assisted by the reactions of the lateral muscles. 
At the junction of the heart with the aorta (figs. 27° 33. t ), as it enters the thorax, 
the last pair of lateral muscles descend forwards, on each side, into that region, and 
thus confine it above the diaphragm (j?). Immediately anterior to the origin of these 
muscles, close to its junction with the aorta, the heart gives off its anterior pair of 
systemic arteries (y), which ramify on the diaphragm and in the posterior parts of 
the thorax. 
Distribution of the Aorta . — The aorta is short, thick, and smooth on its external 
surface, without lateral muscles, or internal divisions into chambers, but there are 
indications of an obliterated chamber at its commencement. It descends obliquely 
forwards and downwards, and after passing beyond the great median arch of the 
thorax, to which many of the muscles of this region of the body are attached, it is 
widened, and rests on the upper surface of the oesophagus, and gives off the vessels 
to the head, to the organs of locomotion, and to form the great spinal artery ; at this 
part of its course its distribution is exceedingly interesting. I have already attempted 
to show the remarkable uniformity of principles on which the nervous and circulatory 
systems are developed. In no instance is this uniformity more curiously illustrated 
than in the distribution of the aorta to the limbs and to the head in the aggregation 
of segments that constitute the cephalothorax. We have seen that vessels are given 
off from corresponding parts of the chambers of the heart both in the Myriapoda and 
the Scorpion, and that these vessels, the systemic arteries , are given to precisely 
similar parts in both. In the Myriapoda the anterior pair of these vessels form a vas- 
cular collar around the oesophagus in the posterior region of the head, and this also 
is the case in the Scorpion. The median continuation of the vessel beyond this collar 
in the Myriapoda is given to the head, to the brain, optic nerves, antennae and in- 
ternal parts of the mouth ; while the external parts of the manducatory organs, the 
great foot-jaws or mandibles, are supplied from the vascular collar, or from parts im- 
mediately connected with it. Now, notwithstanding the aggregation of all these parts 
