EXTRKMITY. 



101 



IH.IIIUII of the articular cylinder is raised con- 

 siderably above it. This arrangement accounts 

 for llii' hollow angle manifest on the outer side of 

 the rlhoxv-joint when the forearm is extended. 



\\ e have yet to describe two proee-,x<-s 

 which are connected in great measure with the 

 outer and inner extremities of the articular cy- 

 linder, and to which we have already referred, 

 as lieing the points in which the margins of 

 the bone terminate. The external one is trian- 

 gular and thick, rough upon its surface, and 

 projects slightly. It is improperly called the 

 external enndyle more correctly it should be 

 designated epicondyle, being applied to the 

 outer surface of what is properly the external 

 comlyle. This process affords attachment to 

 the external lateral ligament of the elbow- 

 joint and to the principal supinator and ex- 

 tensor muscles on the forearm, whence it has 

 been called condi/liu c.rtensurius. The inter- 

 nal process is very prominent, distinctly trian- 

 gular, terminating the inner edge of the hu- 

 nierns and connected with the trochlea; it is 

 more correctly denominated epitruchtca. It 

 atiiirds insertion to the internal lateral ligament, 

 and to the pronator and flexor muscles of the 

 forearm. Its posterior surface is slightly hol- 

 lowed at the line of its junction with the rest 

 of the bone ; the ulnar nerve passes behind it. 



The humerus is the principal lever of the 

 pectoral extremity ; hence in all animals its 

 strength is proportionate to the force and power 

 which is required in the limb. In the ele- 

 phant it is a massive pillar of support; and 

 here we may notice a variety following the 

 same law which influences the difference in 

 the aspect of the glenoid cavity of the scapula, 

 already noticed ; namely, that the angle be- 

 tween the axes of the head and shaft of the 

 humerus, is at its maximum when the arm- 

 bone is mainly an instrument of support, and 

 diminishes as that bone is more used for pre- 

 hension and other purposes ; and as this use is 

 found for this bone chiefly in the human sub- 

 ject, we may presume that in man the angle 

 in question is the least removed from a right 

 angle. When this limb is used mainly for 

 support and progression, a considerable range 

 of motion in the shoulder-joint is not required, 

 the tuberosities at the upper extremity of the 

 bone project and limit the motions of the joint. 

 \\ lien, however, a considerable motion is ne- 

 cessary, these tubercles are depressed as in man, 

 so as not to interfere with these motions. The 

 lower extremity of the humerus likewise affords 

 marks indicative of the mobility of the fore- 

 arm and hand ; thus, in the one case one or 

 both of the edges of the bone which terminate 

 in the epitrochlua and epicondyle are promi- 

 nent ana strong in proportion as the muscles 

 which arise from it are frequently called into 

 play, as when the pronating and supinating 

 motions of the forearm are extensive : in the 

 other case this ridge is imperfectly developed, 

 and the principal modification of the lower end 

 of the bone is to be seen in the articular cy- 

 linder, where greater depth is given to the 

 trochlea, in order to aflbrd increased strength 

 and security to the elbow, joint. 



VOL. II. 



Om; of the most singular instances of the 

 developement of bony pioeesie-, in accordance 

 with muscular power is in the case of the mole. 

 In this little animal the whole anterior ex- 

 tremity is constructed entirely with reference 

 to its burrowing habits; its short, thick, and 

 almost square clavicle and its elongated lever- 

 like scapula tend to the same end, as its amaz- 

 ingly strong humerus. The upper extremity 

 of this latter bone is extremely broad ; it pre- 

 sents two articular surfaces, being articulated 

 with the clavicle as well as with the scapula, 

 and the tuberosities which give insertion to 

 the muscles of rotation are enormously de- 

 veloped. The body of the bone is short, thick, 

 and strong ; the inferior extremity is nearly as 

 large as the superior ; both the epicondyle and 

 epitrochlea are very highly developed, especially 

 the latter, which is accounted for by the fact that 

 the muscles of pronation are those most called 

 into action, in order to enable the animal to 

 employ the accessory bone on the radial side of 

 the hand, in scraping up the earth. This 

 large size of the humerus, and great develope- 

 ment of its muscular eminences, is found in all 

 fossorial animals, as the megatherium, the pan- 

 golins, beavers, ant-eaters, moles, and mono- 

 tremata. In the two last the developement is 

 the most remarkable. 



In the class of Birds, the humerus is de- 

 veloped as regards the prominence of its mus- 

 cular protuberances, in proportion to the 

 powers of flight. In birds which fly, those 

 eminences are strong and prominent, and the 

 bone itself is proportionally strong; but in those 

 which do not fly, the bone is weak and gene- 

 rally short. In the common pigeon, for ex- 

 ample, the enlargement of the scapular ex- 

 tremity of the humerus, and the developement 

 of the tubercles is very manifest, as well as 

 the strength and thickness of the shaft of the 

 bone. 



Structure. The structure of the humerus is 

 characteristic of that of long bones in general. 

 In a vertical section we observe that the re- 

 ticular texture is chiefly accumulated towards 

 the extremities; the shaft being mainly formed 

 of compact tissue. At the upper extremity we 

 notice the mark of union of the epiphysis of 

 the head, which corresponds to the line of the 

 anatomical neck of the bone. The canal, when 

 a transverse section of it is viewed, appears 

 somewhat quadrilateral in form. Its walls are 

 formed of very dense compact tissue. 



Developement. The ossification of the hu- 

 meius begins in its shaft, and that very early, 

 according to Meckel about the second month ; 

 the shaft goes on enlarging, but the extremities 

 are still cartilaginous during the whole of in- 

 tra-uterine life, and for the first year after birth. 

 The superior extremity is developed by two 

 points of ossification, one for the head, the 

 other for the great tuberosity ; about the be- 

 ginning of the second year the ossification of 

 the head of the bone commences, and from 

 the four-and-twentictli to the thirtieth month 

 the ossification of the great tuberosity begins. 

 A i ( 'Mi-ding to Bfolard, a small ossific point for 

 the lesser lulx-ro.sity is visible in the fifth or 



