. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. RESPIRATION. 289 arc placed, like the leaves of a book, in close apposition with each other, the unattached sides floating freely in the water, the current of which, as it traverses the branchial pas- sages, bearing directly on the flat surfaces or the leaflets, separates them effectually from each other. Viewed edgewise, as represented in the following sketch, the arrangement of these respiratory membranous extensions may be more fully understood. On the branchial processes of the eel these leaflets amount to 700 in num
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. RESPIRATION. 289 arc placed, like the leaves of a book, in close apposition with each other, the unattached sides floating freely in the water, the current of which, as it traverses the branchial pas- sages, bearing directly on the flat surfaces or the leaflets, separates them effectually from each other. Viewed edgewise, as represented in the following sketch, the arrangement of these respiratory membranous extensions may be more fully understood. On the branchial processes of the eel these leaflets amount to 700 in number ; on those of the turbot, to 900 ; on those of the cod, to 1000; on those of the salmon, to 1-400. Especial attention is invited to Jig. 237. It shows that only the mucous membrane (/. c. the layer of epithelium and basement membrane) is doubled upon itself, so as to assume the form of folds, the stratum of capillary blood being single. From this beautiful arrange- ment it results that the blood, in its passage through the labyrinth of this plexus, must present two sides of an extremely divided stream to the agency of the circumfluent medium. If the network of vessels were duplicated upon a supporting basis, only one side of the sheet of blood could re- ceive the influence of the surrounding me- dium. Regarded mechanically, such a plan would present little of the delicacy and per- fection which really distinguishes this most elaborate specimen of organised structure. This singlc-\&\er disposition of the respiratory vessels, doubling thus the surface of exposure, obtains as the universally prevailing type of structure in the breathing organs of aquatic animals. It is an arrangement which faci- litates in a very remarkable manner the inter- change of gases between the blood and the water. The capillary vessels of the branchiae of fishes, in internal diameter, exceed very little the long axis of the blood-corpuscle. The internal calibre of these channels varies from -g^Q
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