202 OSTEOLOGY. | ’ 
meet, the angle is blunt, and has a narrow facet which articulates with the semi- 
lunar. Jnferiorly there are two articular facets separated by a ridge; these are 
shehtly concave from be- 
fore backwards, and are for 
articulation, the outer with 
the fourth, and the inner 
~~ _IV. Metacarpal With the fifth metacarpal 
| bone. The dorsal surface, 
more or less triangular in 
shape, is rough for liga- 
ments. 
The unciform articulates 
with five bones—viz. the os 
magnum, semilunar, cunei- 
form, and the fourth and 
fifth metacarpals. 
Semi- 
lunar 
THE CARPUS AS A WHOLE. 
Cuneiform, When the carpal bones 
arearticulated together they 
form a bony mass, the dorsal 
surface of which is convex 
from side to side. Anteri- 
4 Os magnum 
4 
~ Semilunar orly they present a grooved 
appearance, Goncave from 
Fic. 148.—THE RicHr Uncirorm Bone. side to side. This arrange- 
ment is further emphasised 
by the forward projection on the inner side of the pisiform and hook of the 
unciform, whilst externally the tuberosity of the scaphoid and the ridge of the 
trapezium help to deepen the furrow by their elevation. To these four points the 
anterior annular ligament of the wrist is attached, which stretches across from side 
to side, and thus converts the furrow into a canal through which the flexor tendons 
pass to reach the fingers. 
Architecture.—The bones are formed of fairly compact spongy tissue, surrounded by a thin 
shell of denser bone. They are very vascular, and their non-articular surfaces are pierced by 
many foramina. 
Variations.—Increase in the number of the carpal elements is occasionally met with, and 
these have been ascribed to division of the scaphoid, semilunar, cuneiform, os magnum, trape- 
zoid, and unciform. In the last-mentioned case the hook-like process persists as a separate 
ossicle ; but the researches of Thilenius (Morph. Arbeiten, Bd. v. Heft 3, S. 462), together with the 
observations of Pfitzner, prove that all these supernumerary bones are but the persistence of inde- 
pendent cartilaginous elements which are met with in the hand of the human embryo between 
the second and fourth months, and which either disappear or become fused with adjacent 
elements. Of these the most interesting is the os centrale, first described by Rosenberg, and 
subsequently investigated by Henke, Leboueq, and others. This is met with almost invariably 
as an independent cartilaginous element during the earlier months of foetal life, and occasionally 
becomes developed into a distinct ossicle placed on the back of the carpus between the scaphoid 
and os magnum and the trapezoid. Its significance depends on the fact that it is an im- 
portant component of the carpus in most mammals, and is met with normally in the orang 
and most monkeys. Ordinarily in man, as was pointed out by Leboucegq, it becomes fused with the 
scaphoid, where its presence is often indicated by a small tubercle, a condition which maintains 
in the chimpanzee, the gorilla, and the gibbons. 
Further addition to the number of the carpal elements may be due to the separation of the 
styloid process of the third metacarpal bone and its persistence as a separate ossicle. Reduction 
in the number of the carpus has been met with, but this is probably due to pathological causes. 
Morestin (Bull. Soc. Anat. de Paris, tome 71, p. 651), who has investigated the subject, 
finds that ankylosis occurs most frequently between the bases of the second and third metacarpal 
bones and the carpus, seldom or never between the carpus and the first metacarpal, or between .— 
the pisiform and cuneiform. 
Ossification.—At birth the carpus is entirely cartilaginous. An exceptional case is — 
figured by Lambertz, in which the centres for the os magnum and unciform were already 
present. ‘lhe same authority states that it is not uncommon to meet with these centres 
vie 
