THE HUMERUS. 



269 



internal supracondylar ridge. It is usually connected by a fibrous band to the tip 

 of the inner condyle, thus representing the supracondylar foramen found very 

 generally among mammals. The median nerve and generally either the brachial or 

 the ulnar artery pass through it. The process, without any completing ligament. 



We have once seen a bony foramen. 



has been seen hooking over the nerve alone. 



The so-called torsion of the humerus is a very complicated problem arising from the 

 theory of the changes necessary to account for the adult condition of the humerus and femur, 

 assuming them to have been originally symmetrical. The practical point is that the horizontal 

 axis subdividing the articular surface of the head of the humerus, imagined on the same plane 

 as the transverse a.xis of the elbow, forms an angle with the latter. This angle varies consider- 

 ably ; according to Gegenbaur, it is 12° for the adult European. In the lower races it is greater, 

 and still greater in the lower animals. (This is what Continental anatomists call the supple- 

 mental angle, as they assume that the twisting has approached 180°, and that thus the true 

 angle is 168°. We give this as the simplest. ) The angle is greater in the fcetus. Gegenbaur 

 gives it as 59° at from three to four months, and as 34° at from three to nine months, after birth. 

 This change probably occurs in the epiphyses. It is certain that the shaft of the developing 

 humerus does not actually twist, for the borders are straight, as are all the long nerves with the 

 single exception of the musculo-spiral. No spiral fibres have been found in the bone. 



Structure. — The walls of the shaft are of compact bone enclosing a cavity. At 

 the upper end the head is made of round-meshed tissue of considerable density ; the 

 greater tuberosity is of lighter structure ; both are enclosed by thin bone. The line 

 of union of the upper epiphysis is seen on section after it has disappeared from the 

 surface. Transverse sections at the lower end show a system of strong plates passing 

 obliquely from the front to the back above the inner condyle. 



Differences due to Sex, — The chief guides are the greater delicacy of the 

 female bone, and especially the smaller size of the head. It is generally thought 

 that the female humerus presents a sharper angle between the a.xis of the shaft and 

 the transverse axis of the trochlea than does the male, but Berteaux's ^ measurements 

 make the difference too slight to be significant, — 79° for man and 78° for woman. 



Development. — The primary centre for the shaft appears towards the end of 

 the second foetal month, and before birth bone has reached to the extremities, which 

 are formed by the union of several centres. There are two or three for the upper, 

 a chief one for the head coming soon after birth and sometimes earlier. It is 



Ossification of humerus. ^, just before birth; i?, in the first year; Cat three years; C, sections of ends ot 

 preceding; Z), at five years ; E, at about thirteen years; E' , sections of ends of preceding; F, at about sixteen ; F' , 

 sections of ends of preceding, a, centre for shaft ; b, for head ; c, for capitellum and part of trochlea ; d, for greater 

 tuberosity ; e, for head and tuberosities in transverse section ; /", for internal condyle ; g, for inner part of trochlea. 



present at birth in 22.5 per cent, of foetuses weighing seven pounds and over (Spen- 

 cer^). It is almost always present by the end of the third month after birth. In 

 the third year ossification begins in the greater tuberosity, and another point may 

 appear somewhat later in the lesser one. At five all the centres for this end have 



• ' Le Humerus et le Femur, Paris, 1891. 



* Journal of Anatomy and Physiology, vol. xxv., 1891. 



