312 CIRCULATION OF NUTRITIVE FLUID. 



sels that issue from it, and then dilating, to receive a fresh supply from 

 the vessels that pour their contents into it. It seems quite certain, 

 however, from the extent of the vascular system of these animals, that 

 the influence of such a pulsatile cavity must be quite insufficient to keep 

 up the movement of blood through it. A similar provision is observable 

 in the lower tribes of Worms, in which this contractile vessel lies along 

 the back ; propelling the blood forwards, by a sort of peristaltic move- 

 ment, through trunks which pass out at its anterior termination ; and 

 receiving it again after it has circulated through the system, by vessels 

 which enter at its posterior extremity. In the higher orders of Worms, 

 in the Myriapoda or Centipede tribe, and in Insects, we find this dorsal 

 vessel divided by transverse partitions containing valves, into separate 

 cavities which answer to the different segments of the body. Each of 

 these is, to a certain extent, the heart of its own segment, receiving and 

 propelling blood by trunks which open into it ; but they all participate 

 in the more general circulation just described, a large portion of the 

 blood being poured into the hindermost segment, transmitted forwards 

 from cavity to cavity through the valves which separate them, and at 

 last propelled through trunks that issue from the most anterior segment. 

 In some instances we find that two or three of these trunks, on either 

 side, pass round the oesophagus, and reunite below it, so as to enclose 

 it in a sort of collar and they form a main trunk by this union, which 

 runs backwards along the under surface of the body, and which distri- 

 butes the blood to its different organs by lateral branches. These 

 subdivide into a capillary network, and the returning vessels, which 

 originate in this network, pour the blood which has circulated through 

 it into the posterior cavity of the dorsal vessel. Still it is very 

 evident from the observation of the circulation in those transparent 

 species in which the whole process can be distinctly watched under the 

 Microscope, that the contractile, power of the dorsal vessel is far from 

 sufficient of itself to sustain the Circulation ; and that the movement of 

 the blood through the capillary network is in part due to forces deve- 

 loped during its progress, being often retarded or accelerated in parti- 

 cular spots, without any visible change in the propelling force of the 

 central organ. Moreover, the blood, during some part of its course, 

 almost always escapes from the proper vessels into lacunce channelled 

 among the tissues ; and over its flow through these, no central impelling 

 organ can have much influence. 



553. In most of these animals, there are distinct organs of Respira- 

 tion, confined to some one part of the body ; and we often find that the 

 vessels which convey blood to them, are furnished with distinct con- 

 tractile portions, like so many supplementary hearts, for the purpose of 

 propelling the blood through them more energetically. In proportion 

 as we ascend the series of Articulated animals, do we find for the most 

 part, a more vigorous and regular circulation, both for the nutrition of 

 the system, and for the transmission of the blood through the respira- 

 tory organs ; but there is an exception in the case of insects, which 

 deserves special notice. In this class, the circulation is much less vigo- 

 rous than it is in other Articulated animals of similar complexity of 

 structure ; though it might have been anticipated, that the extraordinary 



