THE CIRCULATION OF THE BLOOD. 9921 
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. _ 
CIRCULATION 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 isa 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 prop- 
er, muscular tissue (itself elas- 
tic), fibrous tissue, and a flat epi- 
thelial lining, so smooth that the 
friction therefrom must be mini- 
mal 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 different from 
all the other blood-tubes in that 
they possess valves opening to- 
ward the heart throughout their 
course. The veins are at least 
twice as numerous as the arte- 
ries, and their capacity many 
times greater. The small vessels 
or capillaries are so abundant 
and wide-spread that, as is well 
known, the smallest cut any- ; 
where gives rise to a flowof blood, Mi elipwere (anton). 6. endotheliana 
owing to section of some of these ise minscular fers of the middie coat; 
tubes, which, it will be remem- Gr Mavaatitsa S57 OF the outer coat, 
bered, are not visible to the un- 
aided eye. It is estimated that their united area is several 
hundred (500 to 800) times that of the arteries. 
