brought about. (r.r.r. in the Figure mark ribs.)

Section 42. The oxygen and carbon dioxide are not carried in
exactly the same way by the blood. The student will know from his
chemical reading that neither of these gases is very soluble, but
carbon dioxide is sufficiently so in an alkaline fluid to be conveyed
by the liquid plasma. The oxygen however, needs a special portative
mechanism in the colouring matter of the red corpuscles, the
haemoglobin, with which it combines weakly to form
oxy-haemoglobin of a bright red colour, and decomposing easily in
the capillaries (the finest vessels between the arteries and veins), to
release the oxygen again. The same compound occurs in all true
vertebrata, and in the blood-fluid of the worm; in the crayfish a similar
substance, haemocyanin, which when oxygenated is blue, and when
deoxydized colourless, discharges the same function.

Section 43. The blood returns from the lungs to the left auricle (l.au.)
by the pulmonary veins, hidden in the Figure by the heart, passes
thence to the thick-walled left ventricle (l.vn.), and on into the aorta
(ao.).

Section 44. The beating of the heart is, of course, a succession of
contractions and expansions of its muscular wall. The contraction,
or systole, commences at the base of the venae cavae and passes
to the auricles, driving the blood before it into the ventricles, which
then contract sharply and drive it on into the aorta or pulmonary
artery; a pause and then a dilatation, the diastole follows. The flow of
the blood is determined in one direction by the various valves of the
heart. No valves occur in the opening of the superior cavae but an
imperfect one, the Eustachian valve, protects the inferior cava; the
direction of the heart's contraction prevents any excessive back-flow
into the veins, and the onward, tendency is encouraged by the suck
of the diastole of the ventricles. Between the left ventricle and auricle
is a valve made up of two flaps of skin, the mitral valve, the edges of
the flaps being connected with the walls of the ventricle through the
intermediation of small muscular threads, the chordae tendinae,
which stretch across its cavity to little muscular pillars, the papillary
muscles; these attachments prevent the mitral valve from flapping
back into the auricle, and as the blood flows into and accumulates in
the ventricle it gets behind the flaps of the valve and presses its
edges together. When the systole of