V. On the Morphology of the Muscles on the Extensor Aspect of the Middle and 
Distal Segments of the Limbs: with an Account of the various Paths which 
are adopted by the Nerve-Trunks in these Segments. 
By H. St. Jouy Brooks, M.D., Ohief Demonstrator of Anatomy in the 
University of Dublin. 
Part I.—On tHE Extensor Muscizs in cerTAIN AMPHIBIANS, REPTILES, 
AND MAMMALS. 
Tuis paper forms the first or introductory part of a systematic study of the 
extensor muscles in the groups of Vertebrata above fishes. In the succeeding part 
I hope to take up, one by one, the various individual muscles, and to trace the 
anatomy of each through a series of types upwards from the Amphibia, and, in fact, 
to discuss the history of the whole group of muscles. In this preliminary paper 
I offer a revised account of the muscles of this region in the three remarkable 
forms—Hatteria, Menobranchus, and Ornithorhynchus, of which specimens were 
furnished me for the purpose by Professor D’Arcy THompson. 
Hatteria. 
In the forelimb of Hatteria, a superficial stratum of extensor muscles arises 
from the radial condyle of the humerus as an unsegmented mass, from which the 
triceps is completely separate. Passing downwards, this divides longitudinally into 
three sectors—a radial, an ulnar, and an intermediate. The radial sector presents 
(1) a tolerably distinct fasciculus (Fig. 9, ew. cr.) inserted into the radial carpal 
bone, and obviously corresponding to the eat. carpi radialis longior et brevior of 
Mammals; and (2) a larger mass (s) which clothes the whole posterior and outer 
aspects of the shaft of the radius, and represents the supinator longus et brevis. 
The ulnar sector is partly inserted into the upper part of the shaft of the ulna, 
constituting an anconeus™ (); and partly into the base of the fifth metacarpal 
(ext. carpi ulnaris) by a short tendon, from which an aponeurotic slip is given off 
to the fifth belly of the extensor brevis digitorum. ‘he extensor longus digitorum 
(ex. c.d.) constitutes the intermediate sector, and is situated partly superficial to 
the other two. It passes at the wrist into an aponeurotic tendon, dividing into 
four slips, each of which is inserted in the bases of two adjacent metacarpal bones, 
*The anconeus is usually regarded as a part of the triceps. In Hatteria the triceps is perfectly 
segmented from all the muscles in the forearm, including the anconeus. I shall give reasons, in the second 
part of this Paper, which will show, I believe conclusively, that the anconeus of Human Anatomy is to be 
associated with the extensor carpi ulnaris, and not with the triceps. 
