The Nerve Bundle via the Median Nerve Innervating the Intrinsic Muscles of the Hand in a Gorilla
By Hidehiko Saito, MD
Emanuel Kaplan stated, “The anatomical variations in the human obtain great practical meaning for the anthropologist and the surgeon, when compared with the anatomical architecture of the great apes.” The author was fortunate to have the opportunity to perform the anatomical dissection of the right forelimb of a female western gorilla (Gorilla gorilla), estimated to have been 40 years old, that died at Ueno Zoo in Tokyo.
All intrinsic muscles but three thenar muscles and radial two lumbricals are innervated by one of three main branches of the median nerve. In this specimen of a gorilla, the median nerve trifurcates at the level of the distal border of the pronator teres. The most ulnar one is the branch concerned, tentatively called the ulnar antebrachial branch of the median nerve (UABM).
The UABM runs ulno-distally and comes close to, but not united with, the ulnar nerve at the ulnar side of the wrist. It bifurcates there, and the ulnar bundle goes distally to the radial border of the hypothenar muscles, and then further bifurcates, one being the ulnar digital nerve of the little finger and the other the 3rd common digital nerve. The radial bundle goes radially as the deep branch of the ulnar nerve does in the human hand. It first gives off a branch to the hypothenar muscles and then branches to the lumbrical muscles of the little and ring fingers, and then to both dorsal and palmar interossei in the 4th and 3rd intermetacarpal spaces. It goes further radially to give branches to both the transverse and the oblique heads of the adductor pollicis (ADP). The terminal branch of this bundle goes through the interval between those two heads of ADP and comes out to the postadductor space to innervate both dorsal and palmar interossei in the 2nd intermetacarpal space and finally both heads of the 1st dorsal interosseous in the 1st intermetacarpal space.
The UABM was then followed proximally by intraneural dissection to find that most fasciculi of this branch come from the medial cord and some from the lateral cord. Intraneural dissection of the medial cord revealed that one half of fasciculi going to UABM come from the medial division of the middle trunk・C7 root (Eisler) and the other half from the lower trunk・C8 root. The ulnar nerve emerges at this level and is composed of some fasciculi of the medial division of the C7 root and some from the C8 root. The ulnar nerve contains motor fasciculi going to the flexor carpi ulnaris (FCU) and very few motor fasciculi to the intrinsic muscles through a thin communication branch going to the radial bundle of the UABM in the forearm. The dissection of the distal part of the ulnar nerve showed three fasciculi at the level of the wrist. All three are sensory branches. The first terminates in the fatty tissue at the proximal border of the hypothenar muscles, the second extends along the ulnar border of the hand and the third extends dorsally underneath the FCU tendon.
Hidehiko Saito, MD, Hamamatsu, Japan