THE CIRCULATION OP THE BLOOD. 219 



the latter being carried too far toward the auricles, the pocket- 

 ing action of the semilunar valves with their strong margin 

 and meeting nodules (corpora Arantii) ; the relative thickness 

 of auricles and ventricles, and the much greater thickness of 

 the walls of the left than of the right ventricle— differences, 

 which are related to the work these parts perform. 



The latter may be well seen by making transverse sections 

 of the heart of an animal, especially one that has been bled to 

 death, which specimen also shows how the contraction of the 

 heart obliterates the ventricular cavity. 



It will also be well worth while to follow up the course 

 of the coronary arteries, noting especially their point of 

 origin. 



The examination of the valves of the smaller hearts of cold- 

 blooded animals is a matter of greater difficulty and is facili- 

 tated by dissection under water with the help of a lens or dis- 

 secting microscope ; but even without these instruments much 

 may be learned, and certainly that the valves are relatively to 

 those of the mammalian heart imperfectly developed, will be- 

 come very clear. 



CIRCUIiATIpN OF THE BLOOD IN THE MAMMAL. 



It is highly important and quite possible in studying the 

 circulation to form a series of mental pictures of what is trans- 

 piring. It will be borne in mind that there is a set of elastic 

 tubes of relatively thick walls, standing open when cut across, 

 dividing into smaller and smaller branches, and finally ending 

 in vessels of more than cobweb fineness, and opening out into 

 others, that become larger and larger and fewer and fewer, till 

 they are gathered up into two of great size which form the right 

 auricle. The larger pipes consist everywhere of elastic tissue 

 proper, muscular tissue (itself elastic), fibrous tissue, and a flat 

 epithelial lining, so smooth that the friction therefrom must be 

 minimal as the blood flows over it. 



The return tubes or veins are like the arteries, but so thin 

 that their walls fall together when cut across. They are differ- 

 ent from all the other blood-tubes in that they possess valves 

 opening toward the heart throughout their course. The veins 

 are at least twice as numerous as the arteries, and their capacity 

 many times greater. The small vessels or capillaries are so 

 abundant and wide-spread that, as is well known, the smallest 



