200 MUSCLES OF THE UPPER LIMB. 



the upper arm. It takes origin in an extended line wliicli may be divided 

 into three portions, viz., an anterior from the front of the external third 

 of the clavicle, a middle from the outer edge and point of the acromion, 

 and a posterior from the lower border of the spine of the scapula as 

 far back as the triangular surface at its inner end. The fasciculi from 

 these several parts converge as they descend, and are inserted by a 

 strong thick tendon into the deltoid impression, a triangular rough 

 surfoce above the middle of the humerus, on its outer side. 



The three parts, comiiosing the deltoid muscle, differ somewhat in the disposi- 

 tion of the muscular fasciculi and the tendons. The anterior and posterior 

 portions are composed of long fibres which, arising directly from the clavicle and 

 spine of the scapula, are inserted into the outer sua-face of the marginal parts of 

 the inferior tendon. In the acromial portion, while some of the muscular fibres 

 spring directly from the bone, yet most of them arise in a pennifonn manner 

 from the sides of three or, more frequently, foui- tendinous septa, which pass 

 downwards into the substance of the muscle. These oblique fibres, running nearly 

 parallel to each other, are inserted in a similar manner into the sides of two, or 

 more frequently three, septa which pass from below upwards and alternate with 

 the upper septa. The oblique fibres, from the outside of the first and fourth upper 

 septa are inserted into the marginal parts of the main tendon. There are besides 

 wedge-shaped bundles of muscular fibres, rising directly from the acromion in 

 the intervals between the pennifonn bundles, which, are inserted into the tips of 

 the lower tendinous septa, and others which springing from the ends of the 

 upper tendinous sej^ta, are directly inserted into the humerus between the lower 

 septa. The main tendon of insertion spreads fi'om below upwards for some 

 distance on the deep surface of the muscle. 



JicIatio>ix. — At the posterior border of the deltoid, the aponeurosis covering the 

 infraspinatus muscle divides into two thin layers, of which one covers the muscle 

 superficially, and the other lines its deep surface. At its anterior border this 

 muscle is in contact with the pectoralis major, the cephalic vein lying between 

 them. In immediate contact with the deep surface is the large bursa, which 

 separates this muscle and the acromion from the shoulder joint and the muscles 

 suppoi-ting it. The deltoid muscle covers the origins of the biceps, coraco- 

 brachialis, and long head of the triceps, and the insertions of the subscapularis, 

 supraspinatus. infraspinatus and teres minor muscles, as also the circumflex 

 vessels and nei-ve. 



Varieties. — The deltoid muscle is not subject to great varieties. One of the 

 commonest is a greater degree of subdivision of its parts than usual. Another 

 is the continuation into it of fibres from the trapezius, as in animals wanting the 

 clavicle. Occasionally its anterior part is closely united with the great pectoral 

 muscle. Its insei-tion varies sometimes in extent. A remarkable prolongation of 

 its tendon on the radial border of the forearm, as far as the insertion of the 

 supinator longus, seems to repeat the tensor plicae alaris of the bird. (Macalister). 



The supraspinatus muscle arises from the whole of the supraspinous 

 fossa of the scapula to within a short distance of the neck of the bone, 

 and from an aponeurosis by which it is covered. Its fibres converge beneath 

 the acromion to a tendon, which adheres to the capsule of the shoulder- 

 joint, and to the tendon of the infraspinatus muscle, and is inserted 

 into the upper of the three facets on the great tuberosity of the humerus. 



The infraspinatus muscle is of a triangular form, and occupies the 

 greater part of the infraspinous fossa. It arises from the fascia covering 

 it, from the under surface of the spine, and from tlie dorsal surface of 

 the scapula in the fossa, except those parts at the lower angle and along 

 the external border, to which the teres muscles are attached. The fibres 

 converge to a tendon, which, concealed at first within the substance of 

 the muscle, is inserted into the middle facet of the great tuberosity of 

 the humerus. 



