456 CEPHALOPODA. 



stances, from the peculiar muscular structure of the vein at the part 

 where these orifices occur, their use appears to depend on, or to be 

 in connection with, a power of regulating their diameters.* 



(498.) The blood derived from the great venous receptacles 

 (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 (Jig. 211, 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. 



In Sepia there is appended to each lateral heart a fleshy appen- 

 dage (TTI, w), which, however, is not met with in the generality of 

 Dibranchiate Cephalopods. These bodies are attached to the 

 hearts by narrow pedicles, and Professor Owen considers them to 

 be rudiments of the additional pair of branchiae met with in the 

 Pearly Nautilus. 



In Nautilus Pompilius the hearts just mentioned do not exist ; 

 doubtless, because the greater extent of surface afforded by the 

 four branchiae of this Cephalopod renders the presence of extra- 

 ordinary agents for impelling the blood through them, in order 

 to ensure efficient respiration, unnecessary. 



After undergoing exposure to the surrounding medium in the 

 extensive ramifications of the branchial arteries, the purified blood 

 is returned to the organs belonging to the systemic circulation. 

 In Sepia it is first received from the branchiae by two dilated 

 sinuses (z, t), which might almost be regarded as systemic auricles ; 

 and from these it passes into a strong muscular cavity (/r), which 

 corresponds in function with the left ventricle of the human heart, 

 and by its pulsations forcibly propels the blood through all the 

 arterial ramifications of the vascular system. Two aortae, one de- 

 rived from each of its extremities, arise from the systemic ventricle, 

 the commencement of each being guarded by strong valves so dis- 



* Opuscit. p. 30. . . :>'.* 



