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160 ANATOMY.

body are produced. It is familiarly known as "flesh," and it is dis-
tributed over the entire framework of the body and in the coats of
blood-vessels and the viscera.
In the higher vertebrates the color of the muscular tissue is generally
red, varying in shade, however, according to the locality in which it is
found and to other circumstances. The voluntary muscles, called into
most constant action, are deeper in color than others of their class. A
marked illustration of this is shown in the pectoral muscles of the bird
and the common fowl. The former, being called into almost constant
use in the act of flying, are dark in color, while the latter, being almost
wholly inactive, are extremely light in shade.
Muscular tissue also constitutes a large proportion of the weight of
the human body. It has been estimated by Liebig that a man weighing
150 pounds is 27 pounds skeleton, 60 pounds viscera (with skin, lat,
blood, etc.), 63 pounds muscle.
Each muscle constitutes a separate organ, and either acts independ-
ently or in conjunction with other muscles as accessories. The great
vital property of muscular tissue is contractility, which power is
excited to activity by the influence of various stimuli.
The greater number of the muscles of the body, such as those of
locomotion, respiration, mastication, the first part of deglutition, expres-
sion, etc., are compelled by the will acting through the nerves with
which the parts are supplied. These are known as voluntary muscles.
Others, again, as those of the intestinal canal and the vascular system,
cannot be brought into action by the force of the will. These are called
involuntary muscles, and are controlled by the sympathetic nervous system.
These two classes of muscles differ also in their histological construc-
tion, and will therefore be considered separately.
The Voluntary Muscles are fibres appearing under the microscope trans-
versely striated, and generally oblong in shape. Usually, tendons are at-
tached to their extremities, by means of which they are united to bones, and
sometimes to other tissues ; for instance, the sphincter muscle of the mouth.
The fleshy part of a muscle is called its belly, and its terminal pro-
lono-ations are its tendons of origin and insertion. The term origin
of a muscle or tendon generally applies to that extremity which is
stationary, and the term insertion is applied to the more movable
point. Example : the origin of the temporal muscle is that portion
arising in the temporal fossa on the side of the head, while its insertion
is at the coronoid process of the inferior maxilla. The skull is the fixed
point of the muscle, while the loM'er jaw is the movable one. This
rule is not without exceptions, as in certain localities the fixed point of
a muscle may become the movable point. The origin of a muscle may
be large and its insertion small, and vice versa; or the origin and inser-
tion may be of equal size.
A thorough knowledge of the origin and insertion of muscles is
absolutely necessary to a full understanding of the mechanical action
of the parts to which they are attached. It is essential in diagnostica-
tino- fractures and dislocations. A knowledge, also, of the direction of
the fibres composing muscles, and of the relation of muscles to adjoining
parts, enables the surgeon to locate disease, and serves as a guide to the