it contains a greater quantity of lime. The _Muscular Tissue_ is
composed of bundles of fibers, which are enclosed in a cellular
membrane.

[Illustration: Fig. 6.
Human Adipose Tissue.]

Various opinions have been entertained in regard to the formation, or
growth, of bone. Some anatomists have supposed that all bone is formed
in cartilage. But this is not true, for there is an _intra-membranous_,
as well as an _intra-cartilaginous_, formation of bone, as may be seen
in the development of the cranial bones, where the gradual calcification
takes place upon the inner layers of the fibrous coverings.
Intra-cartilaginous deposit is found in the vicinity of the
blood-vessels, within the cartilaginous canals; also, there are certain
points first observed in the shafts of long bones, called _centers of
ossification_. These points are no sooner formed than the cartilage
corpuscles arrange themselves in concentric zones, and, lying in contact
with one another, become very compact. As ossification proceeds, the
cup-shaped cavities are converted into closed interstices of bone, with
extremely thin lamellæ, or layers. These, however, soon increase in
density, and no blood-vessels can be observed within them.

[Illustration: Fig. 7.
Vertical section of cartilage near the surface of
ossification. _1_. Ordinary appearance of the temporary
cartilage. _1_'. Portion of the same more
highly magnified. _2_. The cells beginning to form
into concentric zones. _2_'. Portion more magnified.
_3_. The ossification is extending in the inter-cellular