The levator anguli scapulae is a long strap shaped muscle, situated on the side of the neck, and extending from the superior cervical vertebra to the upper angle of the scapula. Its Origin is by four (sometimes three) ten dinous bundles from the posterior tubercles of the transverse processes of the four superior cervical vertebra ; that which arises from the atlas is the largest ; they are intimately con nected with the splenius colli behind, and with the scaleni before. The fleshy fibres pro ceeding from them unite, and passing down wards, outwards, and backwards, are inserted into the inner surface and posterior margin of the scapula, from its superior angle to near its spine. Here it overlaps a little of. the lesser rhomboid, and is so united with the serratus magnus that Dumeril considers it a portion of this muscle. The dissection of it in some quadrupeds favours this opinion, but in man it appears rather in connexion with the rhom boid.
This muscle is covered by the sterno-mastoid at its upper part, then by the integuments, and afterwards by the trapezius. It rests on the splenius colli, cervicalis descendens, transver salis colli, serratus posticus superior, and lesser rhomboid.
This muscle pulls the superior angle of the scapula upwards and forwards, and by rotating that bone on the thorax becomes a depressor of the shoulder-joint. The rhomboids act with it in depressing the joint ; but the inferior portion of the serratus magnus is its direct antagonist. When the trapezius acts with this muscle, the scapula is drawn directly upwards. If the scapula be fixed, this muscle will incline the neck to its own side.
This muscle undergoes many modifications in the different families of the niammalia. In it is inserted into the spine of the sca pula, not into its angle. In carnivora and rodentia it separates the two portions of the trapezius, and is inserted near the acromial end of the spine of the scapula. In the cat it arises from the basilar process of the os occi pitis and from only one of the cervical vertebra, the atlas. In the horse it does not exist at all. In the dolphin it forms a thin tendon which spreads over the scapula. As to birds and rep tiles, it is replaced in them by other muscles.
Third layer.—Two very thin muscles, the serratus posticus superior and serratus posticus iVerior, constitute the layer.
The serratus posticus superior is quadrilateral. It arises by a thin tendon from the lowest part of the ligamentum nucha, from the last cervi cal and the first two or three dorsal spines.
The fleshy fibres which succeed form a thin plane, pass downwards and outwards, and are inscrtcd by four digitations into the superior border and external surface of the second, third, fourth, and fifth ribs, a little external to their angles.
This muscle is covered by the rhomboid, the trapezius, and, when the shoulder is drawn back, by the serratus magnus. Its origin is united to the two former. It covers the splenii, the longissimus dorsi, transversalis colli, sacro lumbalis and cervicalis descendens ; while on these it is tendinous; then it becomes fleshy and covers the ribs and intercostal muscles. Sometimes it has only three points of insertion. Occasionally we find a bundle of fibres passing from the upper part of this muscle along the levator anguli scapula to be inserted into the transverse process of the atlas.
This muscle elevates the ribs and expands the thorax as in inspiration. It binds down the muscles on which it lies, enabling them to act with more effect.
The serratus posticus Ulterior is very like the last muscle, but a little broader and thinner. It arises from the last two dorsal and first three lumbar spines by a thin tendinous expansion, which is intimately connected with the tendon of the latissimus dorsi, and often destroyed in removing the latter. The fleshy fibres which succeed pass upwards and outwards to be inserted by digitations into the four lowest false ribs. The uppermost digitation is the largest, and is attached to the rib near its angle ; the others become smaller as we descend, and their insertions are more remote from the angles. The lowest is connected with the cartilage of the last rib. This muscle covers the longissimus dorsi and sacro-lumbalis, the ribs and inter costals. It also covers the posterior tendon of the transversalis abdominis, to which it is in separably united.
This muscle draws down the ribs as in ex piration, and binds down the deep lumbar muscles.
A thin semitransparent fibrous layer, called the vertebral aponeurosis, covers the spinal muscles in the interval between the two ser rati. It is continuous with their adjacent edges, and assists them in binding down the long muscles of this region. The fibres of which it is composed pass for the most part trans versely, from the spinous processes to the an gles of the ribs.
These muscles are generally present in the inferior animals, when ribs exist, and have no peculiarity worthy of being noticed here.