86 the skeleton of the horse 



The Third Carpal Bone 



The third carpal bone (Os carpale tertium, os magnum) is much the largest 

 bone of the lower row, forming more than two-thirds of the width of the latter. It 

 is flattened from above downward, and is twice as wide in front as behind. The 

 superior or proximal surface consists of two facets separated by an antero-posterior 

 ridge; the inner facet is concave and articulates with the radial carpal; the outer 

 facet — for the intermediate carpal — is concave in front and convex behind, where it 

 encroaches on the posterior surface. The inferior or distal surface is slightly un- 

 dulating, and articulates almost entirely with the large (third) metacarpal bone, 

 but it usually bears a small oblique facet at its inner side for the inner (second) meta- 

 carpal, and there is commonly a non-articular depression externally. The internal 

 surface faces backward and inward, and bears three facets for articulation with 

 the second carpal, between which it is excavated and rough. The external surface 

 has two facets for articulation with the fourth carpal, and is depressed and rough 

 in its middle. The anterior or dorsal surface is convex and is crossed by a rough 

 transverse ridge. The posterior or volar surface is relatively small, and is rounded; 

 its upper part is encroached upon by the superior articular surface, below which 

 it is rough. 



The Fourth Carpal Bone 



The fourth carpal bone (Os carpale quart um, unciform) is somewhat wedge- 

 shaped, and is readily distinguished from the second by its greater size and its 

 posterior tubercle. The superior or proximal surface articulates with the inter- 

 mediate and ulnar; it is convex and curves outward, backward, and downward, 

 encroaching on the external and posterior surfaces. The inferior or distal surface 

 bears two inner facets for the large (third) metacarpal and an outer one for the ex- 

 ternal (fourth) metacarpal bone. The internal surface has two or three facets for 

 articulation with the third carpal, between which it is excavated and rough. The 

 anterior or dorsal surface is convex and rough. The external surface is small, 

 being encroached upon by the superior articular surface. The posterior or volar 

 surface bears a tubercle on its lower part.' 



The Carpus as a "Whole 



The bones of the carpus, exclusive of the accessory, form an irregular quadran- 

 gular mass, the width of which is about twice the height or the antero-posterior 

 diameter. The anterior or dorsal surface is convex from side to side, depressed 

 along the line of junction of the two rows, and prominent below. The posterior 

 or volar surface is in general slightly convex, but very irregular. It forms with the 

 accessory the carpal groove (Sulcus carpi), which in the recent state is rendered 

 smooth l:)y the posterior ligament; it is converted into the carpal canal (Canalis 

 carpi) for the flexor tendons by the transverse carpal ligament, which stretches 

 across from the accessory bone to the inner side. The proximal surface is widest in- 

 ternally and is elevated in front, concave behind; it is entirely articular and adapted 

 to the carpal articular surface of the radius. The distal surface is also articular and 

 is irregularly faceted in adaptation to the surfaces of the metacarpal bones; each 

 of the lower bones usually articulates with two metacarpal bones, but sometimes 

 the third does not bear on the inner metacarpal bone. The lateral surfaces are 

 both irregular and rough, the internal one lieing the wider. With the excei:)tion of 

 the accessory, ulnar, and second, each bone articulates with two l)ones of the other 

 row. 



Development. — Each ossifies from a single center. 



1 This bone is probably equivalent to the fourth and fiftli earpals of forms in which five 

 carpal elements are present in the lower row. 



