86 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 113. — The left humerus seen from behind (£). 



Fig. 1 14. — The left humerus seen from in front (£). 



Fig. 115. — The head of the left humerus seen from above (f). 



Fig. 116. — The lower end of the left humerus seen from below (|). 



diminution in the thickness of the bone, forming what is called the surgical neck, because the 

 humerus is easily broken in this situation. 



The upper portion of the shaft of the humerus is almost cylindrical, while the lower portion 

 is prismatic and flattened. 



Passing downward from either tuberosity upon the upper portion of the shaft there is a rough 

 ridge; one is the greater tubercular (bicipital) ridge, which gives insertion to the pectoralis major 

 muscle, and the other the lesser tubercular (bicipital) ridge, into which the latissimus dorsi and 

 teres major muscles are inserted. The intertubercular (bicipital) groove is continued down- 

 ward between these ridges for a short distance and they form its lips (Fig. 114). 



Below the greater tubercular ridge upon the outer and posterior portion of the shaft there 

 is a large, flat, roughened surface, the deltoid tuberosity (Fig. 114), for the insertion of the deltoid 

 muscle, and at about the middle of the shaft and upon its inner aspect there is a roughening, 

 which is rarely distinct, for the insertion of the coracobrachialis muscle. Near this rough surface 

 there is a large nutrient foramen (Fig. 114) which leads downward into the bone as the nutrient 

 canal. 



In the lower half of the shaft three surfaces, an antero-intemal, an antero-external, and a 

 posterior, can be recognized. The two anterior surfaces are separated from each other by a flat 

 elevation and from the posterior one by the sharp external and internal borders of the bone (Fig. 

 114). The external border commences below the deltoid tuberosity and is separated from it 

 by a shallow groove for the radial (musculospiral) nerve (Fig. 114). It is sometimes termed the 

 musculospiral groove, and gives attachment by its margins to the outer and inner heads of the 

 triceps muscle. It pursues a spiral course about the middle of the humerus, passing from above 

 downward and from within outward and gradually disappearing below. 



The lower portion of the shaft becomes flatter and broader, and its lateral borders run 

 downward to terminate in two rough projections which are known as epicondyles (Fig. 116), 

 the sharp outer border passing to the small external epicondyle and the inner one to the more 

 prominent internal epicondyle. The posterior surface of the internal epicondyle presents a 

 shallow groove for the ulnar nerve. 



Below the epicondyles is situated the lower articular surface of the humerus (Fig. 116) which 

 articulates with the bones of the forearm. This articular surface presents a separate area for 

 each bone, a large trochlea with a median groove being situated internally for the ulna and a 

 smaller hemispherical capitulum externally for the radius. Above the trochlea and upon the 

 antero-intemal surface at the level of the epicondyle there is a moderately deep depression, which 

 is known as the coronoid fossa (Fig. 114) because it accommodates the coronoid process of the 

 ulna when the arm is flexed, and above the capitulum, upon the lower portion of the antero- 



