298 
MR. NEWPORT ON THE NERVOUS AND CIRCULATORY SYSTEMS 
so well known in insects. The blood, propelled by these successive contractions 
through the aorta, is distributed to the organs in the head and thorax, and the organs 
of locomotion. Part of it also is sent round the aortic arches , through the supra- 
spinal artery, backwards into the abdomen, giving off its minute currents for the 
nourishment of the cord, while another portion intermingled with that collected in 
the portal vessels is sent to the branchiae. But its principal current still flows in the 
spinal artery, along the upper surface of the cord to the terminal ganglion of the tail, 
where it is divided into four streams, two of which go out at the sides of the ganglion 
to nourish the segment, while the other two, now greatly reduced in size, proceed 
backwards along the terminal nerves of the cord, and becoming more and more sub- 
divided in the last segment of the tail are diffused through the surrounding structures. 
These form minute anastomoses with numerous small vessels, which, gradually collect- 
ing in separate trunks on the under surface of the last segment, form the origin of the 
caudal portion of the subspinal vessel, which conveys the returning blood forwards from 
the tail to the abdomen to be aerated in the branchiae before it is again transmitted to 
the heart. In like manner the blood that has already circulated through the organs 
of locomotion, the cephalothorax and abdomen, appears to be collected in the venae 
cavce which transmit it to the branchiae before it is again employed in the circulation. 
Throughout the whole of its course along the artery in the tail the blood is passed in 
small currents, both anterior and posterior to each ganglion, into the subspinal vessel; 
thus intermingling the venous and arterial blood precisely as occurs in the abdomen. 
But the circulation in the caudal prolongation of the heart yet remains to be ex- 
plained. We have already seen that the great dorsal artery in the tail, above the 
colon, forms direct vascular anastomoses around its sides with the subspinal vessel 
on the ventral surface, in which the course of the blood is forwards to the abdomen. 
It is certain, therefore, that the action of the great chamber of the heart must impel 
the hlood at once in every direction, chiefly forwards and laterally, but also in part 
backwards through the caudal artery, otherwise it would be impossible for this struc- 
ture to form its anastomoses with the subspinal vein without occasioning two oppo- 
sing currents in the same vessel, and this diversion of the current may perhaps be 
effected through the interposition of the two imperfect chambers of the heart in the 
last abdominal segment. 
Recapitulation and Conclusion. 
Although I propose to continue these investigations of the anatomy and develop- 
ment of the nervous and circulatory systems in the other classes of Articulata, it may 
be well briefly to recapitulate the principal statements contained in this paper. 
1. First, in regard to the nervous system, w’hich has already so deeply engaged the 
attention of physiologists, certain facts appear, nevertheless, to have been overlooked 
or imperfectly examined. The double nervous cord is composed in all the Articulata 
of a superior and inferior series of longitudinal fibres, superimposed one on the other, 
