. The Bible and science. into theinterior of the claw by means of vessels. But we now find a fluid, blood, which will takeup a much greater quantity of oxygen than ordinarywater can, which can convey it to the muscles of theclaw as well as to all other parts of the body, andthere give it off, taking up in return the carbonicacid formed by combustion in the muscle during itsactivity, and any other products of waste. But thisaction of the blood would soon be exhausted, andit would become so loaded with carbonic acid asto be unfit for its purpose, were there no means of EXTERNAL RESPIRATION -GILL
. The Bible and science. into theinterior of the claw by means of vessels. But we now find a fluid, blood, which will takeup a much greater quantity of oxygen than ordinarywater can, which can convey it to the muscles of theclaw as well as to all other parts of the body, andthere give it off, taking up in return the carbonicacid formed by combustion in the muscle during itsactivity, and any other products of waste. But thisaction of the blood would soon be exhausted, andit would become so loaded with carbonic acid asto be unfit for its purpose, were there no means of EXTERNAL RESPIRATION -GILLS AND LUNGS. 151 renewing the oxygen and removing the acid. Thisis effected by means of branchiae or gills in aquaticanimals, and by air-tubes or lungs in land the branchiae the blood circulates freely, with onlya thin layer of tissue between it and the this tissue oxygen comes from the water,and carbonic acid is given out to it. The same isthe case in the lungs, except that instead of water ^k^. Fig. 74.—Dia^rammatic transverse section through the hinder half of the. body ofa sandworni. to show the arrangement of the vessels, and their connection witlithe branchiae or gills, d, dorsal, v, ventral side, n, ventral medulla, cavity. ?n% branchine. -y, ventral vascular trunk, a. fe, branchial , dorsal vascular trunk, h, branch sun-ounding the enteric canal, v, visceralventral vessel. on the one side of the membrane we have air. Thisexchange of gases between the blood and the wateror air external to the animals body is termedexternal respiration. The blood thus purified passesto the interior of the body, and has there to undergothe changes already described in maintaining thenutrition of the tissues. The interchange of gaseswhich there takes place between it and the tissues, 152 RESPIRATION AND CIRCULATION.
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