A Practical Physiology | Page 7

and
exit of air. In other canals a similar office is filled by this kind of
epithelium.
18. Functions of Epithelial Tissues. The epithelial structures may be
divided, as to their functions, into two main divisions. One is chiefly
protective in character. Thus the layers of epithelium which form the
superficial layer of the skin have little beyond such an office to
discharge. The same is to a certain extent true of the epithelial cells
covering the mucous membrane of the mouth, and those lining the air
passages and air cells of the lungs.
[Illustration: Fig. 5.--Various Kinds of Epithelial Cells
A, columnar cells of intestine; B, polyhedral cells of the conjunctiva; C,
ciliated conical cells of the trachea; D, ciliated cell of frog's mouth; E,
inverted conical cell of trachea; F, squamous cell of the cavity of mouth,
seen from its broad surface; G, squamous cell, seen edgeways. ]
The second great division of the epithelial tissues consists of those
whose cells are formed of highly active protoplasm, and are busily
engaged in some sort of secretion. Such are the cells of glands,--the
cells of the salivary glands, which secrete the saliva, of the gastric
glands, which secrete the gastric juice, of the intestinal glands, and the

cells of the liver and sweat glands.
19. Connective Tissue. This is the material, made up of fibers and cells,
which serves to unite and bind together the different organs and tissues.
It forms a sort of flexible framework of the body, and so pervades
every portion that if all the other tissues were removed, we should still
have a complete representation of the bodily shape in every part. In
general, the connective tissues proper act as packing, binding, and
supporting structures. This name includes certain tissues which to all
outward appearance vary greatly, but which are properly grouped
together for the following reasons: first, they all act as supporting
structures; second, under certain conditions one may be substituted for
another; third, in some places they merge into each other.
All these tissues consist of a ground-substance, or matrix, cells, and
fibers. The ground-substance is in small amount in connective tissues
proper, and is obscured by a mass of fibers. It is best seen in hyaline
cartilage, where it has a glossy appearance. In bone it is infiltrated with
salts which give bone its hardness, and make it seem so unlike other
tissues. The cells are called connective-tissue corpuscles, cartilage cells,
and bone corpuscles, according to the tissues in which they occur. The
fibers are the white fibrous and the yellow elastic tissues.
The following varieties are usually described:
I. Connective Tissues Proper:
1. White Fibrous Tissue. 2. Yellow Elastic Tissue. 3. Areolar or
Cellular Tissue. 4. Adipose or Fatty Tissue. 5. Adenoid or Retiform
Tissue.
II. Cartilage (Gristle):
1. Hyaline. 2. White Fibro-cartilage. 3. Yellow Fibro-cartilage.
III. Bone and Dentine of Teeth.
20. White Fibrous Tissue. This tissue consists of bundles of very
delicate fibrils bound together by a small amount of cement substance.
Between the fibrils protoplasmic masses (connective-tissue corpuscles)
are found. These fibers may be found so interwoven as to form a sheet,
as in the periosteum of the bone, the fasciae around muscles, and the
capsules of organs; or they may be aggregated into bundles and form
rope-like bands, as in the ligaments of joints and the tendons of
muscles. On boiling, this tissue yields gelatine. In general, where white
fibrous tissue abounds, structures are held together, and there is

flexibility, but little or no distensibility.
[Illustration: Fig. 6.--White Fibrous Tissue. (Highly magnified.)]
21. Yellow Elastic Tissue. The fibers of yellow elastic tissue are much
stronger and coarser than those of the white. They are yellowish, tend
to curl up at the ends, and are highly elastic. It is these fibers which
give elasticity to the skin and to the coats of the arteries. The typical
form of this tissue occurs in the ligaments which bind the vertebrae
together (Fig. 26), in the true vocal cords, and in certain ligaments of
the larynx. In the skin and fasciae, the yellow elastic is found mixed
with white fibrous and areolar tissues. It does not yield gelatine on
boiling, and the cells are, if any, few.
[Illustration: Fig. 7.--Yellow Elastic Tissue. (Highly magnified.)]
22. Areolar or Cellular Tissue. This consists of bundles of delicate
fibers interlacing and crossing one another, forming irregular spaces or
meshes. These little spaces, in health, are filled with fluid that has
oozed out of the blood-vessels. The areolar tissue forms a protective
covering for the tissues of delicate and important organs.
23. Adipose or Fatty Tissue. In almost every part of the body the
ordinary areolar tissue contains a variable quantity of adipose or fatty
tissue. Examined by the microscope, the fat cells consist of a number