to escape, and finds no other passage but that made for it in front by the yielding of the valvules which separate it from the second cell. Into this, therefore, it passes; but, at the same time, the preceding dilates, and the blood contained in the intestinal cavity presses against the lateral valvules, which yield and permit it thus to enter by the openings which they protected. The same process is repeated by the second cell, then by the third, and so on; the blood thus traverses them all by regular jerks, without any of them being ever left completely empty."[1]
This process will be better understood from an inspection of the annexed figures, after a drawing by Mr. Bowerbank, with his accompanying explanation.[2] Fig. 1st, a, a, represents two chambers of the dorsal vessel in their greatest state of collapse, when the point of the lower valve is seen closely compressed within the upper one. At the commencement of the expansion, the blood is seen flowing in from the lateral aper-
