THE HUMERUS. 



91 



concealed by the muscles. The coracoid process projects in front beyond the clavicle, 

 and can be readily felt inside the head of the humerus, but can be seen only in thin 

 persons. With the arm hanging by the side, the scapula covers the ribs from the 

 second to the seventh inclusive, sometimes the eighth ; and the root of the spine and 

 the lower edge of the glenoid cavity are on a level with the interval between the 

 third and fourth dorsal spines. It must, however, be remembered that the bone 

 changes its position with every movement of the arm. 



The body of the scapula is in great part thin and translucent, and at these spots contains 

 no cancellated tissue. The head, the coracoid and acromion processes, the prominent border 

 of the spine, and the thick rib along the axillary border, derive their greater thickness and 

 strength from increased thickness of the compact bony substance in some parts, and from 

 cancellated tissue in others. Vascular foramina pierce the upper and lower surfaces of the 

 spine, and others are to be found on the anterior surface of the bone, near the neck. 



The morphological axis of the scapula corresponds to the line of attachment of the spine, 

 extending from the glenoid cavity to the vertebral border of the bone. From this axis three 

 lamina? radiate, the prcscapula, we.wscaj)i(la, and postscapiila, of 

 Parker, giving rise to the three scapular fossa?. The supraspinous 

 angle (between the prescapula and mesoscapula) measures about 

 100 ; the infraspinous and subscapular angles are each about 130 . 1 

 The scapula of man is remarkable for the great development of the 

 postscapula, making the vertebral border very long and the inferior 

 angle prominent. This is an adaptation to the freedom of move- 

 ment possessed by the upper limb. The surface for attachment of 

 the rotator muscles of the humerus is thus increased, and greater 

 leverage is given to the lower part of the serratus magnus muscle, 

 which rotates the scapula upwards. The relative length and 

 breadth of the scapula are expressed by the scapular index = 



100 x breadth 



; r; . The index is higher in the infant than in the 



length 



adult, and in Negroes than in Europeans. (Broca, Bull. Soc. 

 Anthrop. Paris, 1878 ; Flower and Garson, Joum. Anat., xiv ; 

 Turner, " Challenger " Reports, Zoology, xvi ; Dwight, " The Range 

 of Variation of the Human Shoulder-Blade," Amer. Nat., 1887.) 



THE HUMERUS. 



The humerus or arm-bone extends from the shoulder 

 to the elbow, where it articulates with both bones of the 

 forearm. It is divisible into the superior extremity, including 

 the head, neck, and great and small tuber osities ; the shaft ; 

 and the infei'ior extremity, including the external and internal 

 condyles, and the infei'ior articular surface. In general form 

 it is subcylindrical and slightly twisted. 



The superior extremity is the thickest part of the 

 bone. The head is a large hemispherical articular elevation, 

 directed inwards, upwards, and backwards. The neck, as 

 described by anatomists, is the short portion of bone which 

 supports the head ; inferiorly, it passes into the shaft ; 



superiorly, it is a mere groove between the head and the great tuberosity. 

 The great tuberosity is a thick projection, continued upwards from the external 

 part of the shaft, and reaching nearly to the level of the upper margin of the 

 head; it is surmounted by three flat surfaces, the uppermost of which gives 

 attachment to the supraspinatus muscle, the lowest to the teres minor, and the 

 intermediate one to the infraspiuatus muscle. Separated from the great tuberosity 

 by the commencement of the bicipital groove is the small tuberosity, oval and 

 prominent ; it looks forwards and gives attachment to the subscapularis muscle. 



POSTSCAPULA 



Fig. 89. VERTICAL SEC- 

 TION OF THE SCAPULA, 

 PASSING THROUGH THE 

 MIDDLE OP THE THREE 

 FOSSJ3. (G. D. T.) \ 



1 See Ward's " Outlines of Human Osteology." 



