nerves should be completely removed and the arrangement of the brachial plexus studied.

The Brachial Plexus (plexus brachialis) is formed by the anterior primary divisions of the four lower cervical nerves and the greater part of the large anterior primary division of the first dorsal nerve. Above, the plexus is further reinforced by a small twig of communication which passes from the fourth to the fifth cervical nerve, whilst below, a similar connecting twig not infrequently passes upwards, in front of the neck of the second rib, from the second to the first dorsal nerve. The manner in which these great nerves unite to form the plexus is very constant. The fifth and sixth nerves unite to form an upper trunk; the seventh remains single and proceeds downwards as a middle trunk; whilst the eighth cervical and first dorsal nerves join close to the intervertebral foramina to constitute a third or lower trunk. A short distance above the clavicle each of these three trunks splits into an anterior and a posterior division. Raise the three anterior divisions on the handle of the knife, and it will be seen that all the three posterior divisions unite to form the posterior cord of the plexus, and, further, that the innermost of these divisions is much smaller than the other two. Of the three anterior divisions the two outer join to whilst the innermost is carried the inner cord of the plexus. the plexus are given off the upper limb.

constitute the outer cord, downwards by itself as From the three cords of branches which supply the

From the above description it will be seen that the plexus, from changes which are effected in the arrangement of its fibres, may be divided into four stages :

Fourth stage,

Three nerve-cords (viz., an outer, inner, and a posterior).

The first two of these stages are generally observed in the lower part of the posterior triangle of the neck, and the two last behind the clavicle and in the upper part of the axilla. It must be understood, however, that the points at which division and union of the different parts of the plexus take place are subject to variation.

Infraclavicular Branches of the Brachial Plexus.-The branches of the brachial plexus are usually classified into two groups, viz., those which arise above the level of the clavicle, and those which take origin within the axilla. The latter group of nerves must now be studied by the dissector of the upper limb. They consist of a number of short branches, which end in the muscles forming the anterior and posterior walls of the axilla, and a series of large terminal branches, which are prolonged downwards into the upper arm. They are:

1. Axillary branches

From outer cord:

External anterior thoracic.

From inner cord :

Internal anterior thoracic.

From posterior cord :

Three subscapular nerves.

2. Brachial branches

From outer cord:

Musculo-cutaneous.

Outer head of median.

From inner cord:

Inner head of median.

Ulnar.

Internal cutaneous.

Lesser internal cutaneous.

From posterior cord:

Circumflex.

Musculo-spiral.

The Anterior Thoracic Nerves (nervi thoracici anteriores)

are the branches of supply to the pectoral muscles, or, in other words, to the two muscles which form the anterior wall of the axilla. The external anterior thoracic nerve springs from the outer cord of the plexus, pierces the costocoracoid membrane above the level of the pectoralis minor, and breaks up into branches which sink into the deep surface of the pectoralis major. The internal anterior thoracic nerve, somewhat smaller, arises from the inner cord of the plexus, and passing forwards between the axillary artery and vein enters the deep surface of the pectoralis minor. After supplying this muscle its terminal filaments emerge from its anterior surface, and sink into the pectoralis major. The pectoralis major is therefore supplied by both anterior thoracic nerves; the pectoralis minor by the internal anterior thoracic nerve alone. Close to their origin the two nerves are usually united by an arch or loop, thrown over the front of the axillary artery; in other cases they may join in a plexiform manner, before proceeding to their destinations.

Subscapular Nerves (nervi subscapulares).-The three subscapular nerves spring from the posterior cord of the plexus, and supply the three muscles which form the posterior wall of the axilla. The upper subscapular nerve is placed high up in the axilla. It is very short, sometimes double, and it sinks into the substance of the subscapularis muscle. The long or middle subscapular nerve accompanies the subscapular artery and supplies the latissimus dorsi. The lower subscapular nerve gives twigs to the lower border of the subscapularis muscle, and ends in the teres major.

Dissection. The cords of the brachial plexus may now be divided. Begin with the inner and outer cords, because when these are thrown downwards a better view of the posterior cord and the three subscapular nerves will be obtained. When the posterior cord is cut the arm should be forcibly dragged away from the trunk, so as to put the serratus magnus on the stretch.

Posterior Thoracic Nerve (nervus thoracicus longus). -The external respiratory nerve of Bell, or the posterior

thoracic nerve, as it passes downwards upon the outer surface of the serratus magnus, may now be studied in its whole length. It is the nerve of supply to the serratus magnus, and it arises in the root of the neck by three roots from the brachial plexus. The upper two roots (one from the fifth cervical and the other from the sixth cervical nerve) pierce the scalenus medius, and uniting into one stem give off branches to the upper part of the serratus magnus. The third root takes origin from the seventh cervical nerve, and passes in front of the scalenus medius. It runs downwards for a considerable distance on the surface of the serratus magnus, before it unites with the other part of the nerve. The entire nerve, thus formed, can be followed to the lower part of the serratus, giving twigs to each of its digitations.

Serratus Magnus Muscle.-The serratus magnus arises by fleshy digitatións from the upper eight or nine ribs, about midway between their angles and cartilages. These slips are arranged on the chest-wall, so as to present a gentle curve convex forwards. The lower three or four interdigitate with the external oblique muscle of the abdomen. The serratus magnus is inserted into the entire length of the vertebral border of the scapula. The muscle falls naturally into three parts. (a) The upper part, composed of the large first digitation alone, arises from the first and second ribs, and from a tendinous arch between them. The fibres converge, to be inserted into a somewhat triangular surface on the ventral aspect of the superior angle of the scapula. (b) The middle part consists of two digitations which take origin from the second and third ribs. The upper slip is very broad, and springs from the lower border of the second rib. The fibres diverge to form a thin muscular sheet, which is inserted into the anterior lip of the vertebral border of the scapula, between the insertions of the upper and lower portions. (c) The lower part is formed by the remaining digitations of the muscle. These converge to form a thick mass, which is inserted into a rough surface upon the ventral aspect of the

inferior angle of the scapula. The deep surface of the serratus magnus is in contact with the chest wall.

Removal of the Arm from the Body.-Draw the arm forcibly from the side and cut through the serratus magnus, the omo-hyoid, and the latissimus dorsi muscles, also the suprascapular artery and nerve, and the vessels and nerves in relation to the trapezius and rhomboids, if these have not been previously divided. The arm will then be found to be free, and it may be carried to one of the tables which are reserved for the dissection of separate parts.

SHOULDER-SCAPULAR REGION.

In the dissection of this region the following parts must be studied :

1. Cutaneous nerves of the shoulder.

2. Deep fascia.

3. Deltoid muscle.

4. Sub-acromial bursa.

5. Anterior and posterior circumflex vessels.

6. Circumflex nerve.

7. Dorsalis scapulæ artery.

8. Subscapularis muscle.

9. Supraspinatus, infraspinatus, teres minor, and teres major

muscles.

10. Bursæ in connection with the shoulder-joint.

11. Suprascapular nerve and artery.

12. Acromio-clavicular joint, and the coraco-acromial arch.

Muscles Inserted into the Clavicle and Scapula. The insertions of the muscles which have already been divided should first engage the attention of the student. These should be carefully defined and the precise extent of each studied. Begin with the omo-hyoid, which springs from the superior border of the scapula: then deal in the same way with the levator anguli scapula, rhomboideus minor and major, which are attached to the vertebral border of the bone, and the serratus magnus, which is inserted into the ventral aspect of the superior and inferior angles, and the intervening portion of the vertebral border of the scapula. The insertion of the pectoralis minor into the coracoid process, and of the trapezius into both clavicle and scapula should also be thoroughly examined. When this has been done these

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