BACK, MUSCLES OF THE.—The mus cles of the back are very numerous and complex. There is much variety in their origins and inser tions in different subjects, and in many cases it is not easy to decide with which of two adjoining muscles we are to connect certain bundles of fibres ; a distinct impression, therefore, is not always obtained from an examination of the part, nor will a repetition of the dissection pre sent us with the same view in another subject. Hence it happens that anatomists differ as to the number of muscles to be met with, some dividing into two or more muscles what others consider as one ; this proves another source of difficulty. The names and the enumeration of them, as given by Albinus, we shall follow pretty closely : we esteem them the best on the whole, and they have the advantage of being generally adopted in these countries : viz. the trapezius or cucullaris, latissmus dorsi, rhom boideus major, rliomboideus minor, levator an guli scapula, serratus posticus superior, serratus posticus inferior, splenius capitis, splenius colli, sacro-lumbalis, longissimus dorsi, spinalis dorsi, semi-spinalis dorsi, cervicalis descendens, trans versalis colli, trachelo-mastoideus, complexus, spinalis multifidus spiaice, intcr-spinales, inter-trausversales, rectus capitis posticus major, rectus capitis posticus minor, obliquus capitis in frrior, and obliquus capitis superior. These muscles are placed in pairs, one on each side of the median line ; none of them can be said to be exactly in the middle. We shall examine them in the order they present themselves to us in dissecting.
We find these muscles disposed in layers, and each layer differing from the others in the shape or use of the pieces which compose it. Six such layers may be enumerated. The first consists of the trapezius and latissimus dorsi, muscles somewhat triangular in form, and destined to act principally on the upper extre mity. The second consists of the rhomboidei and levator anguli scapula. These are qua drangular, approaching a square shape, and act on the scapula. The third layer is formed of the serrati, of similar shape, but acting on the ribs. The jburth consists of the splenii ; these, more elongated than the last, rotate and erect the head and neck. The fift h layer is com posed of very long muscles, acting chiefly as erectors of the spine and head, viz. the sacro lumbalis, longissimus dorsi, spinalis, and semi spinalis dorsi, cervicalis descendens, transver salis colli, trachelo-mastoideus and complexus. The sixth layer, again, is formed of short mus cles, rotating and erecting the head or minute portions of the spinal column ; these are the recti and obliqui of the head, the spinalis colli, inter-spinales, inter-transv ersales, and multifid us spina.
First layer.—The trapezius and latissimus dorsi, which form the first layer, almost com pletely conceal all the other muscles of this region, and in superficial extent are scarcely succeeded by any two muscles in the body.
The trapezius is thin, triangular, and very extensive. One of its surfaces is turned to the integuments, and covered by the superficial fascia, and by a fine layer of cellular tissue which closely adheres to it. The trapezius arises from the internal third of the superior oblique ridge of the os occipitis, from the liga mentum nuchaa, and from the spinous pro cesses of the last cervical and of all the dorsal vertebra. The superior fibres run downwards, outwards, and a little forwards, the middle transversely, and the inferior upwards and out wards ; all converge, and are inserted into the external third of the posterior border of the clavicle, the acromio-clavicular ligament, the acromion process, the upper edge of the spine of the scapula, and the tubercle which termi nates this spine at the base.
The origin of this muscle is by tendinous fibres which are from half an inch to an inch long in the occipital portion ; in the cervical they are very short until we come down to the sixth cervical vertebra, where they begin to lengthen ; at the first dorsal they are an inch and a half in length, again they diminish, and at the fourth dorsal spine they are scarcely to be seen ; but at the tenth they again increase in length, and form a triangular tendon. It some times happens that this muscle has no connexion with the eleventh and twelfth dorsal vertebra. The long tendinous fibres of the two trapezii, at the junction of the cervical and dorsal re gions, form an oval aponeurosis of considerable size, called the cervical aponeurosis, which is supposed to give greater strength to this part. All the spinal origin has its fibres blended with those of the opposite muscle, and supraspinal ligament. The insertion is by a mixture of tendinous and fleshy slips, except at the extre mity of the spine of the scapula, where a little tendon is formed which glides over a small triangular surface to be inserted into the top of the tubercle. The plane which this muscle forms is curved on the side of the neck, and its fibres are there a little twisted. Instead of three sides this muscle has actually five : 1st, a superior ; 2nd, an internal—these are its ori gins; 3rd, an external, which is its insertion, and two others which are unconnected, viz. 4th, an inferior external, and 5th, a superior external. Of these the first is so short that it attracts no notice ; the other four are of un equal lengths—hence the name trapezius. But the third and fourth sides are so nearly in one continuous line that the whole muscle appears triangular.