34 



they breathe, and the degree of muscular effort that may at any time have been 

 expended. 



In other classes of animals subject to the same mutations of surrounding 

 pressure, various structures have been recognised as accommodating the san- 

 guiferous system to these changes ; as, for example, the extraordinary rete mira- 

 bile in the intercostal spaces of the Cetacea*, and the varied muscular and elastic 

 powers connected with the branchial artery of fish, which, according to Sir 

 Everard Homef, bear a relation to their powers of descending to great depths. 

 The auricle also in fish, and the capacious venous sinuses which terminate in 

 it, must afford convenient receptacles to the blood when in a state of expansion, 

 or prevented by any cause from flowing freely through the gills ; and tlie val- 

 vular structures for obviating the regurgitation of the contents of the ventricle 

 into the auricle, or of the auricle into the sinus, are more complete in this 

 than in any other class of vertebrate animals. But the branchial ventricle, in 

 those Cephalopods which possess it, is unprovided with an auricle ; and the 

 Pearly Nautilus, if we except the follicles appended to the vessels passing to 

 the gills, has no receptacle connected with the vascular system adapted to the 

 uses above mentioned. I am therefore induced to believe that these follicles 

 reUeve the vascular system, by affording a temporary receptacle for the blood 

 whenever it accumulates in the vessels, either from the effects of a general ex- 

 pansion, or of a partial impediment in its course through the respiratory organs, 

 and that they serve to regulate the quantity of blood sent to these organs. 



After communicating with the follicles the branchial arteries continue their 

 course outwards, and having reached the roots of the branchiae, they become 

 contracted in size ; and at this part is situated a valve (8. pi. 6.) in each, which 

 opposes the retrogression of the blood. Immediately beyond this valve each 

 artery enters separately the root of the branchiee of its respective side, and then 

 dilates into a wider canal (9. pi. 6.), which is continued through the soft white 

 fleshy substance (r. pi. 6.) forming the central stem or support of the branchia. 

 A vein which returns the blood from the shell-muscle also penetrates on each 

 side the muscular root of the branchia;, and terminates at the commencement of 



* J. Hunter, Observations on Whales, Pliilos. Trans. Ixvii. p. 415. 

 t Philos. Trans, ciii. p. 234. 



