me, and it is then said to be calcified. This
calcified cartilage then undergoes absorption-- it must not be
imagined for a moment that bone is calcified cartilage. Simultaneous
with the formation of the cavities (s.) due to this absorption,
connective tissue (p.c.i.) from the surrounding perichondrium (p.c.)
grows into the ossifying* bar. It is from this connective tissue that the
osteoblasts (o.b.) arise, and bone is built up. Throughout life a bone is
continually being absorbed and reformed by the activity of the
osteoblasts. An osteoblast engaged in the absorption instead of the
formation of bone is called an osteoclast.

* The formation of bone is called ossification. To ossify is to become
bony.

Section 72. The great thing to notice about this is that cartilage does
not become bone, but is eaten into and ousted by it; the osteoblasts
and osteoclasts replace entirely the cartilage corpuscles, and are
not derived from them.

Section 73. We may mention here the structure of the spleen
(Figure 1, Sheet 1). It consists of a connective tissue and muscular
coating, with an internal soft matrix much resembling botryoidal
tissue, traversed by fibrous trabeculne (= beams, planks) containing
blood-vessels, and the whole organ is gorged with blood, particularly
after meals. The consideration of its function the student may
conveniently defer for the present.

Section 74. Here also, we may notice the lymphatics, a series of
small vessels which return the overflow of the blood serum from the
capillaries, in the nutrition of the tissues in all parts of the body,
to the thoracic duct (see Section 36), and the general circulation. At
intervals their course is interrupted by gland-like dilatations, the
lymphatic glands, in which masses of rapidly dividing and growing
(proliferating) cells occur, of which, doubtless, many are detached and
become, first "lymph corpuscles," and, when they reach the veins,
white blood corpuscles.

5. _The Skeleton_

Section 75. We are now in a position to study the rabbit's skeleton.
We strongly recommend the student to do this with the actual bones
at hand-- they may be cleared very easily in a well-boiled rabbit. This
recommendation may appear superfluous to some readers, but, as a
matter of fact, the marked proclivity of the average schoolmaster for
mere book-work has put such a stamp on study, that, in nine cases
out of ten, a student, unless he is expressly instructed to the
co