14 H. ST. J. BROOKS ON THE 
Deep layer.—The ext. brevis digitorum appears, at first sight, to be wanting. We 
find, however, an ext. ossis metacarpi pollicis and an ext. secundi internodii pollicis, 
which differ from the muscles which I have described under these names in Hatteria 
only by being situated a little higher up in the forearm, and possessing relatively 
longer tendons. A third member of the group (ext. indicis) which is partly ulnar 
and partly carpal in origin in Hatteria, arises in Man entirely from the ulna. 
Occasionally, an ext. medii digiti is found arising from the ulna immediately below 
the preceding. It appears, therefore, that these four muscles have ascended from 
the carpus to the forearm bones, the most radially placed (ext. oss. met. poll) having 
ascended first, and the others followed in their order from the radial to the ulnar 
side.* It appears probable that when this stage is reached all the available space on 
the back of the forearm bones is taken up, and when the time comes for the bellies 
of the ext. brevis which appertain to the ring and little fingers to ascend in their 
turn, they creep upwards on the aponeurotic covering of the four radially placed 
muscles, and seek a bony origin from the external condyle of the humerus. 
Being here greatly compressed by the superjacent ext. communis digitorum and ext. 
carpi ulnaris they are forced to the surface between these two muscles and become 
the ext. minimi digiti of Human Anatomy, a muscle which, occasionally in Man and 
normally in many of the Mammalia, gives off two tendons, one for the little finger 
and one for the ring finger. This muscle has been almost universally regarded as 
belonging to the superficial stratum. I shall give (in Part II.) a summary of my 
reasons for classing it with the deep stratum. I may mention here, however, that 
although in most cases the muscular part appears to be quite superficial, the annular 
tendon (when developed) always crosses on the deep surface of the tendons of the 
common extensor, thus retaining, in the hand, a trace of its primitive position as a 
member of the deep extensor stratum. 
The metacarpal heads of the short extensor are not represented in the normal 
human subject. They appear, however, as varieties, and have been described under 
the name of ext. brevis digitorum manus. It is generally implied that this muscle 
is homologous to the ext. brevis digitorum pedis, and this is expressly stated to be 
the case by Macatister.t A comparison with Hatteria and other reptilian and 
amphibian forms shows, however, that the ext. brevis digitorum of the human foot 
is serially homologous to the deep layer of muscles in the forearm, which have 
vacated their primitive position on the carpus and ascended to the middle segment 
of the limb. The ext. brevis digitorum manus corresponds to the metacarpal heads 
* This ascent has already commenced in Hatteria, but the greater part of the deep extensor is still 
carpal in origin in that reptile. In many Amphibia (Menopoma, Cryptobranchus) the muscle is placed at 
a much more distal point of the limb, and arises almost entirely from the carpus. For the sake of simpli- 
city, the eat. primi internodii pollicis is omitted from the above account—it is merely a slip of the ext. 
ossis metacarp? pollicis. 
+Macauister. Annals and Mag. of Nat. Hist. Vol. I.—Fourth Ser., 1868., p. 315. 
