582 CEPHALOPODA. 



view, these follicles relieve the vascular system by affording a tempo- 

 rary receptacle for the blood whenever it accumulates in the vessels, 

 owing to a partial impediment to its course through the respiratory 

 organs, serving in this manner to regulate the quantity of blood sent to 

 the branchiae*. 



(1557.) In Nautilus, Professor Owen found, in addition to the spongoid 

 appendages connected with the veins, lodged in what he denominates 

 the "pericardium" that the great trunk of the vena cava itself presents 

 a structure precisely analogous to what has been already described 

 when speaking of the venous system of Aplysia among the GASTEBOPODA 

 ( 1420), namely a free communication between the interior of the vein 

 and the cavity of the peritoneumf. The vein is of a flattened form, 

 being included between a strong membrane on the lower or ventral 

 aspect, and a layer of transverse muscular fibres which decussate each 

 other on the upper or dorsal aspect. The adhesion of the coats of the 

 vein to the muscular fibres is very strong ; and these fibres form in 

 consequence part of the parietes of the vein itself throughout its whole 

 course. But there are several small intervals left between the muscular 

 fasciculi and corresponding round apertures both in the vein and in the 

 peritoneum ; so that the latter membrane at these points seems to be 

 continuous with the lining membrane of the vena cava. The distin- 

 guished anatomist referred to counted as many as fifteen of these 

 openings, and most of them were sufficiently large to admit the head of 

 an eye-probe. Here, therefore, as in Aplysia, there are direct com- 

 munications between the interior of the vena cava and the great serous 

 cavity of the abdomen ; and moreover, in both instances, from the pecu- 

 liar muscular structure of the vein at the part where these orifices occur, 

 their use appears to depend on, or to be in connexion with, a power of 

 regulating their diameters J. 



(1558.) The blood derived from the great venous receptacles (fig. 

 290, d d) is at once conveyed to the branchiae, and distributed through 

 all the lamellae ( g g) which enter into the composition of the respiratory 

 apparatus. Two distinct hearts, one placed on each side of the body, 

 are interposed between the branchiae and the great trunks of the venous 

 system, serving by their action forcibly to drive the blood through the 

 ramifications of the branchial arteries. These lateral hearts (fig. 290, e e) 

 are of a blackish colour, and their walls moderately thick : internally 

 their cavities are filled with intercommunicating cells ; and moreover a 

 strong mitral valve is placed at the orifice through which they receive 

 blood from the veins, as well as smaller valvules at the origin of the 

 branchial arteries .; the latter enter the principal stem of the branchiae, 

 and, running beneath the ligament (/), divide and subdivide, so as to 

 be dispersed over all the branchial leaflets. 



* Memoir on Nautilus Pompilius, p. 34. 



t Op. tit. I Op. cit. p. 30. 



