THE HUMERUS 121 



enter the foramina -wliieh cluster around the circumference of the head and tul)er- 

 osities. .Vt the top of the l^icipital groove there is a large nutrient foramen, which 

 transmits a branch from the anterior circumflex artery. The nutrient artery of the 

 shaft is derived from a muscular In-anch of the brachial; it enters the bone near the 

 middle of the inner 1)order inmiediately l)elo\v the insertion of the coraco-brachialis, 

 and is directed to the distal end. The lower extremity is nourished by numerous 

 twigs derived from the anastomotic, the superior and inferior profunda, and the 

 recurrent branches of the radial, ulnar, and interosseous arteries. 



Ossification. — The humerus ossifies from one primary and six secondar}' 

 nuclei. The centre for the shaft appears about the eighth week of intra-uterine 

 life, and extends very rapidly. At birth the bone presents two cartilaginous 

 extremities, which ossify in the following manner: — A nucleus for the head appears 

 early in the first year; it is not unfrequently present at birth (Spencer). The 

 nucleus for the greater tuberosity appears in the third year. In the fifth year a 

 centre may l^e deposited for the lesser tuberosit}^, but this is not constant. The 

 three nuclei coalesce to form a disc of bone, which unites with the shaft about the 

 twentieth year. 



The inferior extremity ossifies from four centres : the centre for the capitellum 



Fig. 127.— The Head of the Humeeus at the Sixth Year. (Iu section.) 



THE CENTRE FOR THE HEAD APPEARS DURING THE FiRST 



YEAR; IT IS SOMETIMES PRESENT AT BIRTH 



THE CENTRE FOR THE GREATER TUBEROSITY APPEARS IN 



THE THIRD YEAR 



appears in the third 3'ear, and those for the inner (ulnar) condyle, the trochlea, 

 and external (radial) condyle at the fifth, tenth, and fourteenth years respectively. 

 The nuclei for the capitellum, trochlea, and radial condyle coalesce before uniting 

 with the shaft, which they do in the seventeenth year. The ulnar condyle joins 

 the shaft somewhat later. 



A study of the upper end of the humeral shaft before its union with the 

 epijihysis is of interest in relation to what is known as the neck of the humerus. 

 The term neck is applied to three parts of this bone. The anatomical neck is the 

 constriction to which the capsular ligament is attached. This is accurately indi- 

 cated by the constriction which lies internal to the tuberosities; the upper extremity 

 of the humeral shaft before its union Avith the epiphysis terminates in a low three- 

 sided pyramid, the surfaces of Avhich are separated from one another by ridges. 

 The inner of these three surfaces underlies the head of the bone, and the two outer 

 surfaces underlie the tuberosities. The axis of the inner, isolated portion forms 

 with the shaft an angle of 130°; it constitutes the morphological neck of the 

 humerus, and is of the same nature as the neck of the femur. The surgical neck 

 is an indefinite area below the tuberosities where the lione is liable to fracture. 



