. The microscope and its revelations. gill of the eel, showing the branchiallamellae : A, portion of one of these pro-cesses enlarged, showing the capillarynetwork of the lamella?. by the minute subdivision ofthe cavity not being here neces-sary. In the frog (for example)the cavity of each lung is un-divided ; its walls, which aivthin and membranous at thelower part, there present asimple smooth expanse; and itis only at the upper part, wherethe extensions of the trachea!cartilage form a network overthe interior, that its surface isdepressed into sacculi whoselining is crowded with blood-vesse
. The microscope and its revelations. gill of the eel, showing the branchiallamellae : A, portion of one of these pro-cesses enlarged, showing the capillarynetwork of the lamella?. by the minute subdivision ofthe cavity not being here neces-sary. In the frog (for example)the cavity of each lung is un-divided ; its walls, which aivthin and membranous at thelower part, there present asimple smooth expanse; and itis only at the upper part, wherethe extensions of the trachea!cartilage form a network overthe interior, that its surface isdepressed into sacculi whoselining is crowded with blood-vessels (fig. 800). In thismanner a set of air-cells isformed in the thickness of theupper wall of the lung, whichcommunicate with the generalcavity, and very much increasethe surface over which the blood comes into relation with the air ;but each air-cell has a capillary network of its own, which lies011 one side against its wall, so as only to be exposed to the airon its free surface. In the elongated lung of the snake the same. FIG. 800.—Interior of upper part of lung of frcg. 1064 VERTEBKATED ANIMALS general arrangement prevails ; but the cartilaginous reticulationof its upper part projects much farther into the cavity, and inclosesin its meshes (which are usually square, or nearly so) several layersof air-cells, which communicate, one through another, with thegeneral cavity. The structure of the lungs of birds presents us withan arrangement of a very different kind, the purpose of which is toexpose a very large amount of capillary surface to the influence of theair. The entire mass of each lung may be considered as subdividedinto an immense number of lobules or kinglets (fig. 801. B), each of
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
