The form of the subject or model depends upon :—
1. The shape of the skeleton, which varies with age, race, sex, condition of health, &c.
2. The relative position of the various parts of the skeleton.
3. The condition of the muscles, whether active or passive, and whether well or ill developed.
4. The quantity and quality of subcutaneous fat.
5. The condition of the other coverings—the skin and the hair.
It is necessary for the student to understand various terms used in connection with muscles.
When a muscle contracts it becomes broader, thicker, and shorter: individual bundles of fibres may become prominent, and its tendon, or leader, may stand out as a prominent ridge.
Most muscles are attached to at least two bones. The place of attachment to the more movable bone is called the muscle’s insertion, and to the more fixed bone, its origin.
In the limbs the more movable bone is usually, but not always, distal, i.e. more removed from the centre of the body. A very good example of the application of these terms is to be found in the latissimus dorsi. This muscle’s chief action is to pull the arm down to the side, and therefore it is said to be inserted into the humerus; but it may be put to quite another use, viz. to pull the side up to the arm, as in climbing.
The skeleton having been dealt with already, let us proceed to examine such of the muscles as are responsible for surface form, and are found in the various regions of the body as soon as the skin and superficial and deep fasciæ have been dissected away. In this examination we will note some of the modifications observable in the normal surface form under varying conditions of activity and rest of these same muscles.
For the purposes of anatomical description the regions of the body are classified as follows:—
The upper extremity.
The lower extremity.
The trunk.
The neck.
The head.
Each of these parts presents certain generally recognised subdivision.
The upper extremity. includes :—
The shoulder, the axilla, the arm, the elbow, forearm, wrist, hand, and fingers.
The lower extremity includes :—
The thigh, the buttock, the groin, knee, leg, ankle, foot, and toes.
The trunk includes:—
The spine, the thorax, abdomen, pelvis, and perineum.
The head includes:—
The cranium and the face.
The neck includes all the parts which intervene between the head and the thorax.
Some difficulty arises in fixing the boundaries between the various regions of the body, and between the subdivisions of those regions. Unfortunately for precision of anatomical classification, several important features lie on these boundaries, or cross a boundary and so occur in two or more adjacent regions, but the difficulty thereby created need not trouble the artist.