re modifications of the
areolar.

The _Nervous Tissue_ is of two kinds: The gray, which is pulpy and
granulated, and the white fibrous tissue. The _Adipose Tissue_ is an
extremely thin membrane, composed of closed cells which contain fat. It
is found principally just beneath the skin, giving it a smooth, plump
appearance.

[Illustration: Fig. 5.
Arrangement of fibers in the
Areolar Tissue. Magnified 135 diameters.]

The _Cartilaginous Tissue_ consists of nucleated cells, and, with the
exception of bone, is the hardest part of the animal frame. The _Osseous
Tissue_, or bone, is more compact and solid than the cartilaginous, for
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 lamellae, 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
spaces, and the rows of cells are seen
resting in the cavities so formed, the nuclei being
more separated than above. _3_'. Portion of the
same more highly magnified.]

[Illustration: Fig. 8.
Thigh-bone,
sawn open
lengthwise.]

[Illustration: Fig. 9.
Lower en