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wanting in the teeth of most fishes, snakes, sloths, armadillos, sperm whales, etc.

True dental tissue is confined to vertebrates.

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(6) Adipose Tissue. Certain cells become greatly enlarged and filled with fat, so that the original protoplasm occupies a very small part of the space within the cell membrane. These cells are united into masses by connective tissue, in the skin (as in the "blubber" of whales), between the muscles (as in "streaky" meat), or in

[graphic][subsumed]

FIG. 207.

Fat Cells embedded in Subcutaneous Areolar Tissue. f, fat cells; ", nucleus; c, connective-tissue corpuscles; w, migratory cells; e, elastic fibers; b, capillary blood vessel.

the abdominal cavity, in the omentum, mesentery, or about the kidneys. The marrow of bones is an example. Globules of fat occur in many mollusks and insects; but true adipose tissue is found only in backboned animals, particularly in the herbivorous. In the average man, it constitutes about 6 part of his weight, and a single whale has yielded 120 tons of oil. has the different names of oil, lard, tallow, suet, spermaceti, etc. It is a reserve of nutriment in excess of consumption, serving also as a packing material, and as a protection against cold.

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The fat of animals

(7) Muscular Tissue. If we examine a piece of lean meat, we find it is made up of a number of fasciculi, or bundles of fibers, placed side by side, and bound together

by connective tissue. The microscope informs us that each fiber is itself a bundle of smaller fibers; and when one of these is more closely exam

ined, it is found to consist of a delicate, smooth tube, called the sarcolemma, which is filled with very minute, parallel fibrils, averaging 10000 of an inch in diameter, the whole having a striated aspect and containing numerous nuclei. Tissue of this description constitutes all ordinary muscle, or "lean meat,” and is marked by regular cross lines, or striæ.

Besides this striated muscular tissue, there exist, in the coats of the stomach, intestines, blood vessels, and some

Voluntary

FIG. 208. -
Muscle, portions of two
fibers showing the char-
acteristic transverse mark-
ings; the lighter band is
divided by the row of
minute beads constituting
the intermediate disk:
a, termination of muscular
substance and attachment
of adjoining fibrous tissue;
n, nuclei of muscle fibers.

other parts of verte-
brates, smooth muscular fibers, which show
a single nucleus under the microscope, and
do not break up into fibrils (Fig. 319).
The gizzards of fowls exhibit this form.

All muscle has the property of shortening itself when excited; but the contraction of the striated kind is under the control of the will, while the movement of the smooth from the heart of fibers is involuntary.72 Muscles are well

FIG. 209.-Striated
Muscular Fibers,

Man, divided by

transverse septa supplied with arteries, veins, and nerves;

into separate nu

cleated portions; but the color is due to a peculiar pigment, much magnified. not to the blood.

Muscular tissue is found in all animals from the coral to man.

(8) Nervous Tissue. Nervous Tissue consists of large,

nucleated cells, which give off one to several processes, the latter serving as paths of communication between the cells themselves or between the cells and the various motor, sensory, and other organs with which they are connected. Such threads of nerve tissue are called

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FIG. 210.
of
Fibers: a, medul-
lary sheath; c, nu-
cleus of neuri-
lemma; R, annu-
lar constriction.

FIG. 211.

Terminal branches

Diagram of a Neuron: a, nerve process; b, neurilemma; c, medullary sheath; d, neurilemma and medullary sheath, combined.

nerve fibers, and each consists essentially of a prolongation of the protoplasmic substance of which the cell body is composed. The cells vary from 0 to 20 of an inch in diameter, and are found in the nerve centers (Fig. 329), the gray portion of the brain, spinal cord, and other ganglia. The fibers vary in structure. In the lowest Metazoa they are merely naked threads of

.

protoplasm. In the higher animals each thread, known as the axis cylinder, is surrounded by a delicate, transparent covering called the neurilemma, analogous to the sarcolemma of muscle tissue. In the vertebrates, the protoplasmic threads found in many parts of the nervous system have an additional covering made of fatty material, which lies between the axis cylinder and the neurilemma, and is known as the medullary sheath. These are called medullated nerve fibers, as distinguished

[graphic]

FIG. 212. - Spinal Ganglion, in longitudinal section, from Cat; the groups of nerve cells lie embedded among the bundles of the nerve fibers.

from the nonmedullated or those which lack the medul- . lary sheath. Fibers of the former kind are found in the white substance of the brain and spinal cord, and run to the muscles and organs of sense. Nonmedullated fibers are found in the gray substance of the nervous system. The axis cylinders are destitute of a sheath in the neighborhood of the cell body. Scattered along the fibers nuclei are found. The large nerve fibers may be 1200 of an inch in diameter, and some are supposed to extend from cell bodies situated in the lower part of the spinal cord down the leg to the foot.

A bundle of nerve fibers surrounded by connective tissue constitutes a nerve in the anatomical sense.

3. Organs and their Functions. — Animals, like plants, grow, feel, and move; these three are the capital facts of every organism. Besides these there may be some peculiar phenomena, as motion and will.

Life is manifested in certain special operations, called functions, performed by certain special parts, called organs. Thus, the stomach is an organ, whose function is digestion. A single organ may manifest vitality, but it does not (save in the very lowest forms) show forth the whole life of the animal. For, in being set apart for a special purpose, an organ takes upon itself, so to speak, to do something for the benefit of the whole animal, in return for which it is absolved from doing many things. The stomach is not called upon to circulate or purify the blood.

There may be functions without special organs, as the amoeba digests, respires, moves, and reproduces by its general mass. But, as we ascend the scale of animal life, we pass from the simple to the complex: groups of cells or tissues, instead of being repetitions of each other, take on a difference, and become distinguished as special parts with specific duties. The higher the rank of the animal, the more complicated the organs. The more elaborated the structure, the more complicated the functions. But in all animals, the functions are performed under conditions essentially the same. Thus, respiration in the sponge, the fish, and in man has one object and one means, though the methods differ. A function, therefore, is a group of similar phenomena effected by analogous structures.

The life of an animal consists in the accumulation and expenditure of force. The tissues are storehouses of power, which, as waste goes on, is given off in various forms. Thus, the nervous tissue generates nerve force; the muscles, motion. If we contemplate the phenomena

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