THE HUMERUS 209 



The Humerus (Arm Bone). 



The humerus (Figs. 207, 208) is the longest and largest bone of the upper 

 extremity; it is divisible into a body and two extremities. 



Upper Extremity. The upper extremity consists of a large rounded head joined 

 to the body by a constricted portion called the neck, and two eminences, the greater 

 and lesser tubercles. 



The Head (caput humeri). The head, nearly hemispherical in form, 1 is directed 

 upward, medialward, and a little backward, and articulates with the glenoid cavity 

 of the scapula. The circumference of its articular surface is slightly constricted 

 and is termed the anatomical neck, in contradistinction to a constriction below the 

 tubercles called the surgical neck which is frequently the seat of fracture. Fracture 

 of the anatomical neck rarely occurs. 



The Anatomical Neck (collum anatomicum) is obliquely directed, forming an 

 obtuse angle with the body. It is best marked in the lower half of its circum- 

 ference; in the upper half it is represented by a narrow groove separating the head 

 from the tubercles. It affords attachment to the articular capsule of the shoulder- 

 joint, and is perforated by numerous vascular foramina. 



The Greater Tubercle (tuberculum majus; greater tuberosity). The greater 

 tubercle is situated lateral to the head and lesser tubercle. Its upper surface is 

 rounded and marked by three flat impressions : the highest of these gives insertion 

 to the Supraspinatus; the middle to the Inf raspinatus ; the lowest one, and the 

 body of the bone for about 2.5 cm. below it, to the Teres minor. The lateral 

 surface of the greater tubercle is convex, rough, and continuous with the lateral 

 surface of the body. 



The Lesser Tubercle (tuberculum minus; lesser tuberosity). The lesser tubercle, 

 although smaller, is more prominent than the greater: it is situated in front, and 

 is directed medialward and forward. Above and in front it presents an impression 

 f 3r the insertion of the tendon of the Subscapularis. 



The tubercles are separated from each other by a deep groove, the intertubercular 

 groove (bicipital groove), which lodges the long tendon of the Biceps brachii and 

 transmits a branch of the anterior humeral circumflex artery to the shoulder-joint. 

 ]t runs obliquely downward, and ends near the junction of the upper with the 

 middle third of the bone. In the fresh state its upper part is covered with a 

 thin layer of cartilage, lined by a prolongation of the synovial membrane of the 

 shoulder-joint; its lower portion gives insertion to the tendon of the Latissimus 

 dorsi. It is deep and narrow above, and becomes shallow and a little broader 

 as it descends. Its lips are called, respectively, the crests of the greater and lesser 

 tubercles (bicipital ridges), and form the upper parts of the anterior and medial 

 borders of the body of the bone. 



The Body or Shaft (corpus humeri) . The body is almost cylindrical in the upper 

 half of its extent, prismatic and flattened below, and has three borders and three 

 surfaces. 



Borders. The anterior border runs from the front of the greater tubercle above 

 to the coronoid fossa below, separating the antero-medial from the antero-lateral 

 surface. Its upper part is a prominent ridge, the crest of the greater tubercle; 

 it serves for the insertion of the tendon of the Pectoralis major. About its center 

 it forms the anterior boundary of the deltoid tuberosity; below, it is smooth and 

 rounded, affording attachment to the Brachialis. 



The lateral border runs from the back part of the greater tubercle to the 



1 Though the head is nearly hemispherical in form, its margin, as Humphry has shown, is by no means a true circle. 

 Its greatest diameter is, from the top of the intertubercular groove in a direction downward, medialward, and back- 



, , 



ward. Hence it follows that the greatest elevation of the arm can be obtained by rolling the articular surface in this 

 direction that is to say, obliquely upward, lateralward, and forward. 

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