These muscles occupy the intercostal spaces, and are attached to the margins of the ribs. There are two layers, crossing like the letter X. The external layer begins at the tubercles and stops at the cartilages: its direction is downwards and forwards. The internal layer begins at the angles and extends to the sternum: its direction is upwards and forwards.

Triangularis sterni.

SO. Inner surface of sternum.

I. Cartilages of second, third, fourth, and fifth ribs.

MUSCLES OF THE BACK.

(Plates XLII. to XLV.)

The plan we propose to adopt in the description of the muscles of the back is to take, first, the more superficial muscles connected with the arm. When these are removed, we bring into view the great muscles of the spine, which fill up the vertebral grooves, and keep the body erect. Lastly, we have the mass of muscles at the back of the neck which are attached to the occipital bone.

*The "ligamenta arcuata" are shown in Plate XLI. The inner of these two arched ligaments extends from the body of the first lumbar vertebra to its transverse process; the outer extends from the transverse process to the last rib.

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THE SUPERFICIAL MUSCLES OF THE BACK.

These are shown in Plate XLII. The most superficial is the trapezius," a triangular muscle of which the limits are defined by the continuous dark line. The other wide-spreading superficial muscle is the "latissimus dorsi." Under the trapezius we have the "rhomboideus" and the "levator anguli scapula" (shown in Plate XLIV.).

When the preceding muscles are removed, we have still to take off the "serratus posticus superior" and "inferior." Understand these belong neither to the arm nor the spine, but to the ribs.

We come now to the great muscles of the spine concerned in keeping the body erect. These are considered to be complicated: the truth is, they are very simple, provided the plan of their arrangement be fairly attended to. An attempt has been made to bring out the plan in Plate XLIII.