atic vessels and that it is a powerful
absorbent.

Absorption is one of the earliest and most essential functions of animal
and vegetables tissues. The simpler plants consist of only a few cells,
all of which are employed in absorption; but in the flowering plants
this function is performed by the roots. It is accomplished on the same
general principles in animals, yet it presents more modifications and a
greater number of organs than in vegetables. While animals receive their
food into a sac, or bag called the _stomach_, and are provided with
absorbent vessels such as nowhere exist in vegetables, plants plunge
their absorbent organs into the earth, whence they derive nourishing
substances. In the lower order of animals, as in sponges, this function
is performed by contiguous cells, in a manner almost as elementary as in
plants. In none of the invertebrate animals is there any _special_
absorbent system. Internal absorption is classified by some authors as
follows: _interstitial_, _recrementitial_, and _excrementitial_; by
others as _accidental_, _venous_, and _cutaneous_. The general cutaneous
and mucous surfaces exhale, as well as absorb; thus the skin, by means
of its sudoriferous glands, exhales moisture, and is at the same time as
before stated, a powerful absorbent. The mucous surface of the lungs is
continually throwing off carbonic acid and absorbing oxygen; and through
their surface poisons are sometimes taken into the blood. The continual
wear and waste to which living tissues are subject, makes necessary the
provision of such a system of vessels for conveying away the worn-out
materials and supplying the body with new.

* * * * *

CHAPTER VI.

PHYSICAL AND VITAL PROPERTIES OF THE BLOOD.

[Illustration: Fig. 38.
Red corpuscles of human blood, represented
at _a_, as they are seen when
rather _beyond_ the focus of the microscope;
and at _b_ as they appear when,
_within_ the focus. Magnified 400 diameters.]

[Illustration: Fig. 39.
Development of human lymph and chyle-corpuscles
into red corpuscles of blood. _A_. A lymph, or white
blood-corpuscle. _B_. The same in process of conversion
into a red corpuscle. _C_. A lymph-corpuscle with the
cell-wall raised up around it by the action of water. _D_.
A lymph-corpuscle, from which the granules have
almost disappeared. _E_. A lymph-corpuscle, acquiring
color; a single granule, like a nucleus, remains. _F_. A
red corpus