parietes on the posterior aspect of the trunk which is uncovered by muscles. Further, between the last rib and the crest of the ilium the anterior border of the latissimus dorsi will generally be observed to overlap the posterior border of the external oblique muscle of the abdominal wall. Sometimes, however, a narrow triangular interval exists between the two muscles, in which is seen a small part of the internal oblique muscle. This space is termed the trigonum Petiti (Fig. 2).

Reflection of the Trapezius.-On the second day the dissector should begin by reflecting the trapezius. This should be done, if possible, in conjunction with the dissector of the head and neck. Divide the muscle about two inches from the spines of the vertebræ, and throw it outwards towards its insertion. The trapezius is very thin at its origin, and the greatest care must therefore be taken not to injure the subjacent rhomboid muscles. The small bursa between the tendon of insertion of the lower part of the trapezius and the triangular root of the spine of the scapula must not be overlooked.

Nerves and Vessels of Supply to the Trapezius.-A dissection of the deep surface of the reflected muscle will reveal the following structures :

a. The spinal accessory nerve.

b. Two or three nerves from the cervical plexus.
c. The superficial cervical artery.

These constitute the nervous and vascular supply of the trapezius.

The nerves have already been displayed by the dissector of the head and neck, as they cross the posterior triangle of the neck. The branches from the cervical plexus come from the third and fourth cervical nerves. On the deep surface of the trapezius they join with branches of the spinal accessory to form the subtrapezial plexus, from which twigs proceed into the substance of the muscle. The terminal twig of the spinal accessory nerve can be traced nearly to the lower margin of the trapezius.

The superficial cervical artery which accompanies the spinal accessory nerve must be followed to the anterior

border of the trapeziu's, where it will be seen to spring from the transversalis colli artery.

Dissection. The posterior belly of the omo-hyoid and the suprascapular artery and nerve can now be displayed by dissecting towards the upper margin of the scapula, and removing carefully the loose fatty tissue in this locality. The dissector of the head and neck must take part in this dissection, and it is well not to expose these structures for more than an inch from the upper margin of the scapula.

Omo-hyoid. Suprascapular Artery and Nerve. The slender posterior belly of the omo-hyoid muscle will be seen to arise from the upper border of the scapula immediately behind the suprascapular notch. It also derives fibres from the ligament which bridges across this notch. The suprascapular artery will be noticed to enter the supraspinous fossa of the scapula by passing over the suprascapular ligament, whilst the suprascapular nerve proceeds into the fossa under cover of that ligament.

Dissection. Draw the scapula well over the edge of the block which supports the chest of the subject. The two rhomboid muscles are thus rendered tense, and the cleaning of the fleshy fasciculi greatly facilitated. The nerve to the rhomboids should be secured at this stage, so that it may be preserved from injury in the further dissection of the region. It can best be detected by dissecting in the interval between the rhomboideus minor and the levator anguli scapulæ about one inch to the inner side of the superior angle of the scapula. It is accompanied by the posterior scapular artery, and it will afterwards be traced upon the deep surface of the rhomboid muscles when they are reflected.

The Rhomboid Muscles constitute a thin quadrangular sheet of muscular fibres, which proceeds from the spinous processes of the vertebræ to the base of the scapula.

The rhomboideus minor is a narrow, ribbon-like fleshy band which runs parallel to the upper border of the greater rhomboid. It springs from the lower part of the ligamentum nuchæ, the spine of the seventh cervical vertebra, and frequently also from the spine of the first dorsal vertebra. It is inserted into the base of the scapula opposite the triangular surface at the root of its spine. It is entirely covered by the trapezius.

The rhomboideus major arises from the upper four or five dorsal spines, and the corresponding part of the supraspinous ligament. Its fibres run obliquely downwards. and outwards, and end in a tendinous cord, which receives insertion into the base of the scapula close to the inferior angle. From this point, up to the commencement of the spine, the tendinous cord is firmly bound to the base of the scapula by areolar tissue. The greater part of the rhomboideus major is covered by the trapezius; only a small portion near the inferior angle of the scapula lies immediately subjacent to the deep fascia.

The Levator Anguli Scapula (musculus levator scapula) is an elongated muscle which arises by four more or less tendinous slips from the posterior tubercles of the transverse processes of the upper four cervical vertebræ, and passes downwards and backwards to be inserted into the base of the scapula from the superior angle to the spine.

Dissection. In cleaning the levator anguli scapulæ muscle care must be taken of the nerves which pass to it from the cervical plexus, and also of the nerve to the rhomboids and the posterior scapular artery which lie under cover of it near the base of the scapula. The dissector of the head and neck has an interest in the levator anguli scapula, and when it has been studied by both dissectors it should be divided midway between its origin and insertion, and the lower portion turned outwards. The nerve to the rhomboids has already been secured in the interval between the rhomboideus minor and the levator anguli scapula, and it has still further been exposed by the reflection of the latter muscle. It may now be displayed in its whole length, together with the posterior scapular artery, which it accompanies, by reflecting the rhomboid muscles. These should be detached from the ligamentum nucha and the vertebral spines, and thrown outwards towards the base of the scapula. In doing this take care of the serratus posticus superior, a thin muscle which lies subjacent, and is apt to be injured.

The Nerve to the Rhomboids (nervus dorsalis scapula) is a long slender twig which arises in the neck from the fifth cervical nerve, and usually in common with the upper root of the nerve of Bell. It pierces the scalenus medius, and then proceeds downwards under cover of the levator anguli scapula to the deep surface of the rhomboid muscles

to which it is distributed. The nerve to the rhomboids likewise supplies one or two twigs to the levator anguli scapulæ.1

The Posterior Scapular Artery is a branch of the transversalis colli, and takes origin in the lower part of the neck close to the outer margin of the levator anguli scapula. At first it proceeds backwards under cover of this muscle, but soon changing its direction it runs downwards along the base or vertebral border of the scapula under cover of the rhomboid muscles. It gives numerous branches to both ventral and dorsal aspects of the scapula, and its terminal twigs may enter the latissimus dorsi. One large branch usually passes backwards in the interval between the rhomboid muscles, or through the greater rhomboid, to reach the trapezius muscle, and another branch, the supraspinal, is given to the supraspinatus muscle, and the structures superficial to it.

Reflection of Latissimus Dorsi.-Divide the muscle by carrying the knife from its upper margin, about three inches from the vertebral spines, obliquely downwards and backwards to a point a little way behind its digitation from the last rib. In raising the inner portion of the muscle care must be taken of the subjacent serratus posticus inferior. The attachment of the latissimus dorsi to the crest of the ilium and to the lumbar aponeurosis can now be verified. The outer part of the muscle is next to be thrown forwards, so that the three costal digitations may be seen from their deep aspect, and also for the purpose of displaying the termination of the subscapular artery and the long subscapular nerve. These are found upon the deep surface of the muscle at the inferior angle of the scapula.

Lastly, replace the outer portion of the latissimus dorsi muscle, and fix it in position by a stitch or two around one or more of the ribs. This is done so as to preserve the posterior fold of the axilla.

The dissector of the arm now stops work for two days. He has completed the dissection of all the dorsal structures which are allotted to him, and he has nothing further to do until the body is turned.

The nerve to the rhomboids sometimes pierces the levator anguli scapulae in two or more branches, which unite in a plexiform manner.

PECTORAL REGION AND AXILLARY SPACE.

On resuming work the dissector will find the body lying upon its back. The chest is raised to a convenient height by means of blocks. A long board is placed under the shoulders for the purpose of supporting the arms when they are abducted from the sides (Fig. 4).

In dissecting the axilla and chest it will be found advantageous if the dissectors of the arm and of the head and neck arrange to work at different hours. The dissector of the head and neck at this stage is engaged at the posterior triangle of the neck, and this dissection cannot be well done unless the arm be placed close to the side and the shoulder depressed. For the dissection of the axilla the arm should be stretched out at right angles to the chest. A compromise between these two positions always results in discomfort to both dissectors.

Four Days are allowed for the dissection of the pectoral region and the axillary space. The arm must then be removed, so as to allow the dissector of the thorax to commence the dissection of the thoracic wall. The following Table may be found useful in regulating the amount of work which should be undertaken each day :

First Day. (a) Surface Anatomy; (b) reflection of skin; (c) cutaneous vessels and nerves of the chest, both on its anterior and lateral aspects; (d) cleaning of the pectoralis major muscle; (e) reflection of the axillary fascia; (ƒ) cleaning of that part of the serratus magnus muscle which lies below the fourth rib.

Second Day.--Dissection of the axillary space from below. This includes the boundaries and contents of the space, in so far as they can be got at without the reflection of any muscle.

Third Day. (a) Reflection of the clavicular portion of the pectoralis major; (b) the costo-coracoid membrane and the structures piercing it; (c) removal of the membrane; (d) the dissection of the upper part of the axilla; (e) reflection of the sternal part of the pectoralis major.

Fourth Day. (a) Reflection of pectoralis minor muscle; (b) general revision of the space and study of the axillary vessels and nerves; (c) removal of the middle third of clavicle; (d) subclavius muscle; (e) brachial plexus; (ƒ) nerve of Bell and serratus magnus muscle; (g) separation of limb from the trunk.

Surface Anatomy.-The entire length of the clavicle can be felt under the skin, and as the student follows its