]40 THE SKELETON. 



to the plane of the glenoid cavity, and must consequently operate most effectively 

 on the head of the humerus, which is contained in that cavity. 



The posterior surface, or dorsum (Fig. 95), is arched from above downward, 

 alternately concave and convex from side to side. It is subdivided unequally into 

 two parts by the spine : the portion above the spine is called the supraspinous 

 fossa, and that below it the infraspinous fossa. 



The supraspinous fossa, the smaller of the two, is concave, smooth, and broader 

 at the vertebral than at the humeral extremity. It affords attachment by its inner 

 two-thirds to the Supraspinatus muscle. 



The infraspinous fossa is much larger than the preceding ; toward its vertebral 

 margin a shallow concavity is seen at its upper part ; its centre presents a promi- 

 nent convexity, whilst toward the axillary border is a deep groove which runs 

 from the upper toward the lower part. The inner two-thirds of this surface 

 affords attachment to the Infraspinatus muscle ; the outer third is only covered by 

 it, without giving origin to its fibres. This surface is separated from the axillary 

 border by an elevated ridge, Avhich runs from the lower part of the glenoid cavity 

 downward and backward to the posterior border, about an inch above the inferior 

 angle. The ridge serves for the attachment of a strong aponeurosis which sepa- 

 rates the Infraspinatus from the two Teres muscles. The surface of bone between 

 this line and the axillary border is narrow in the upper two-thirds of its extent, 

 and traversed near its centre by a groove for the passage of the dorsalis scapulae 

 vessels; it affords attachment to the Teres minor. Its lower third presents a 

 broader, somewhat triangular surface, which gives origin to the Teres major, and 

 over which the Latissimus dorsi glides ; sometimes the latter muscle takes origin 

 by a few fibres from this part. The broad and narrow portions of bone above 

 alluded to are separated by an oblique line which runs from the axillary border, 

 downward and backward, to meet the elevated ridge : to it is attached the 

 aponeurosis separating the tAvo Teres muscles from each other. 



The Spine is a prominent plate of bone which crosses obliquely the inner 

 four-fifths of the dorsum of the scapula at its upper part, and separates the supra- 

 from the infraspinous fossa : it commences at the vertebral border by a smooth, 

 triangular surface, over which the Trapezius glides, separated from the bone by a 

 bursa, and, gradually becoming more elevated as it passes outward, terminates in 

 the acromion process, which overhangs the shoulder-joint. The spine is triangular 

 and flattened from above downward, its apex corresponding to the vertebral 

 border, its base (which is directed outward) to the neck of the scapula. It 

 presents two surfaces and three borders. Its superior surface is concave, assists 

 in forming the supraspinous fossa, and affords attachment to part of the Supra- 

 spinatus muscle. Its inferior surface forms part of the infraspinous fossa, gives 

 origin to part of the Infraspinatus muscle, and presents near its centre the orifice 

 of a nutrient canal. Of the three borders, the anterior is attached to the dorsum 

 of the bone ; the posterior, or crest of the spine, is broad, and presents two lips 

 and an intervening rough interval. To the superior lip is attached the Trapezius 

 to the extent shown in the figure. A rough tubercle is generally seen occupying 

 that portion of the spine which receives the insertion of the middle and inferior 

 fibres of this muscle. To the inferior lip, throughout its whole length, is attached 

 the Deltoid. The interval between the lips is also partly covered by the tendinous 

 fibres of these muscles. The external border, or base, the shortest of the three, is 

 slightly concave, its edge thick and round, continuous above with the under sur- 

 face of the acromion process, below with the neck of the scapula. The narrow 

 portion of bone external to this border, and separating it from the glenoid cavity, 

 is called the great scapular notch, and serves to connect the supra- and infraspinous 

 fossae. 



The Acromion Process, so called from forming the summit of the shoulder 

 (axftov, a summit; oy^oc, the shoulder), is a large and somewhat triangular or 

 oblong process, flattened from behind forward, directed at first a little outward, 

 and then curving forward and upward, so as to overhang the glenoid cavity. Its 



