THE DORSAL NERVES.

They pass forward in the intercostal spaces with the intercostal vessels, being situated below them. At the back of the chest they lie between the pleura and the External intercostal muscle, but are soon placed between the two planes of Intercostal muscles as far as the middle of the rib. They then enter the substance of the Internal intercostal muscles, and, running amidst their fibres as far as the costal cartilages, they gain the inner surface of the muscles and lie between them and the pleura. Near the sternum. they cross in front of the internal mam¬mary artery and Triangularis sterni muscle. pierce the Internal intercostal muscles, the anterior intercostal membrane. and Pectoralis major muscle, and supply the integument of the front of the chest and over the mammary gland, forming the anterior cutaneous nerves of the thorax; the branch from the second nerve is joined with the eupraclavicular nerves of the cervical plexus.

Branches.-Numerous slender muscular filaments supply the Intercostals, the Infracostales, the Levatores costarum, Serratus posticus superior, and Triangularis sterni muscles. Some of these branches, at the front of the chest, cross the costal cartilages from one to another intercostal space.

Lateral Cutaneous .cVerves.-These are derived from the intercostal nerves, midway between the vertebras and sternum; they pierce the External intercostal and Serratus magnus muscles, and divide into two branches, anterior and posterior.

The anterior branches are reflected forward to the side and the fore part of the chest, supplying the integument of the chest and mamma; those of the fifth and sixth nerves supply the upper digitations of the External oblique.

The posterior branches are reflected backward to supply the integument over the scapula and over the Latissimus dorsi.

The lateral cutaneous branch of the second intercostal nerve is of large size, and does not divide, like the other nerves, into an anterior and posterior branch. It is named, from its origin and distribution, the intercosto-humeral nerve (Fig. 413). It pierces the External intercostal muscle, crosses the axilla to the inner side of the arm, and joins with a filament from the nerve of Wrisberg. It then pierces the fascia, and supplies the skin of the upper half of the inner and back part of the arm, communicating with the internal cutaneous branch of the musculo-spiral nerve. The size of this nerve is in inverse proportion to the size of the other cutaneous nerves, especially the nerve of W risberg. A second intercosto-humeral nerve is frequently given off