SACKO-YEETEBEAL ARTICULATION. lo9 



supports the head of the metacarpal bone. In the joint of the 

 thumb there are two sesamoid bones, one situated at each side, which 

 are connected with its ligaments. 



A synovial membrane is present in each joint, and invests the 

 surface of the ligaments which connect the bones. 



The infrrphahmfieal articuhtlons are formed on the same plan as 

 that Avhich obtains in the articulations between the bases of the proximal 

 phalanges with the metacarpal bones. 



Movements of the Wrist and Fingrers. — In the radio-cai-pal and common 

 carpal articulations, there is allowed not only flexion and extension, but a 

 certain amount of lateral bending. The superior articular surfaces of both 

 ranges of carpal bones being prolonged further on the dorsal than on the i^almar 

 aspect, over-extension is allowed in both joints to some degree. In over-exten- 

 sion the opposing sui-faces are most perfectly adapted to each other ; in flexion 

 they are least so. The kind of movement which is allowed between the caiiial 

 and metacaipal bones is best illustrated by placing the hand in such a jjosition 

 that the weight of the body is rested upon the open palm. The metacaipal 

 range, which natiu-ally is concave towards the palm, is flattened : and the inter- 

 osseous and palmar metacaiiial ligaments are thus tightened, while a slight 

 separation of the opposed sui-faces of the bones takes place : so also the palmar 

 caipo-metacarpal ligaments are tightened, and both palmar and interosseous 

 ligaments of the second range of caiT^al bones. The convex part of the os 

 magnum and unciform bone, fitted in these circumstances into the concavity of 

 the first range, is a little wider than the part usually in contact with it ; and 

 thus, while the bones of the fii'st range are separated from the palmar side, those 

 of the second range are pressed still more apart from the distal aspect. The 

 whole 'arrangement secures elasticity. The fourth and fifth metacarpal bones, 

 l;)eing more movable at their caqial articulation than the second and third, bend 

 foi-ward very distinctly in shutting the hand, thus rendering the ^lalm more 

 hollow, and bringing the tijis of the" fingers more closely together. At the 

 l^halangeal ai-ticulations the only movements allowed are flexion and extension, 

 while over-extension is prevented by the ligamentous stmctures in front of the 

 joints. At the metacarpo-phalangeal articulations abduction and adduction 

 are allowed chiefly in the extended position. In the articulation of the meta- 

 carpal bone of the thumb with the trapezium every movement is allowed except 

 rotation, which is prevented by the shape of the articular surfaces. 



ARTICULATIONS OF THE PELVIS. 



Articulatiox of the Pelvis with the last Lumbar Vertebra. 

 — The fifth lumbar is united to the first sacral vertebra by anterior and 

 posterior ligaments of the body, capsular ligaments of the aiilcular 

 processes, ligamenta subflava of the arch, interspinous ligaments, and 

 by an intervertebral plate,"all of which are similar to those between the 

 vertebrae above. It is' also attached to the pelvis by two other liga- 

 ments, as follows. 



The sacro-vertebral ligament extends ol)liquely from the tip of the 

 transverse process of the last lumbar vertebra downwards to the de- 

 pressed lateral part of the base of the sacrum ; its form is triangular, 

 and its fibres diverge as they descend, some of them joining the anterior 

 sacro-iliac ligament. 



The ilio-lnmbar ligament is extended horizontally between the 

 Rumm.it of the transverse process of the last lumbar vertebra and the 

 iliac 2rcst of the innominate bone ; it is inserted into the latter at tho 



