. Text-book of normal histology: including an account of the development of the tissues and of the organs. I. THE CIRCULATORY SYSTEM. The circulatory apparatus comprises the channels for the con-veyance of the blood-stream, the vessels, and the dilated and special-ized portion of the vascular tube, constituting the heart, for the pro-pulsion of the current. In development and structure the severalparts of the vascular system possess much in common, althoughvariations in the details of the walls of the blood-channels suffice todistinguish the different portions. THE BLOOD-VESSELS. The blood-ves
. Text-book of normal histology: including an account of the development of the tissues and of the organs. I. THE CIRCULATORY SYSTEM. The circulatory apparatus comprises the channels for the con-veyance of the blood-stream, the vessels, and the dilated and special-ized portion of the vascular tube, constituting the heart, for the pro-pulsion of the current. In development and structure the severalparts of the vascular system possess much in common, althoughvariations in the details of the walls of the blood-channels suffice todistinguish the different portions. THE BLOOD-VESSELS. The blood-vessels occur in three forms, as arteries, veins, andcapillaries, the latter constituting an expanded system of thin-walled tubules, intimately related to the organs, and especially de-signed to facilitate the interchanges be-FiG. 115. tween the nutritive current which they gZ carry and the tissues through which theypass. The arteries possess three coats—theinner, or intima, the middle, or media,and the external, or adventitia. Sincethese coats vary in relative thickness and Mt%^Wt:i^00^ y\ m Fig. ■ Section of human arterj of mediumsize: /, the intima, consisting of theendothelium (e), the sub-endothelialtissue (f), and the internal elasticmembrane (j-); M, the media, com-posed of the involuntary muscle andthe bundles of elastic tissue (y); A,the adventitia, containing irregularelastic trabeculae iz). Endothelium of artery of frog: the vessel has been treatedwith silver, hence the boundaries of the endothelial plates areindicated by the dark lines of stained cement-substance. Sev-eral pseudo-stomata appear as minute dark areas between thecells. in details of structure with the size of the vessel, it is usual to classify THE CIRCULATORY SYSTEM. gC arteries as small, medium, and large. The first group includes theterminal branches near transformation into capillaries, the second,all the named arteries of the body, except those which, as the aortaor the pulmonary artery, ar
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