MORPHOLOGY OF THE EXTENSOR MUSCLES. 5 
The nervous arrangements in the fore and hind limbs show a very marked 
similarity, especially in regard to the innervation of the short extensor and the 
nerve-supply of the skin of the foot. In two respects, however, considerable differ- 
ence exists: (1) The musculo-spiral (peroneal) nerve in the hind limb winds round 
the fibula; whereas in the fore limb it enters the forearm on the radial side. The 
objections which might be raised on this ground against the homologies of the two 
nerves appear to me to be completely met by the following remarks by Professor 
Humpury. He says :—“ The differences between the musculo-spirals in the two 
limbs, that is, between the peroneal and the radial, are dependent chiefly or entirely 
upon the difference in the direction of the rotation in the two limbs, causing a 
variation in the direction best adapted for the nerve course. . . . For the same 
reason, the nerves to the quadriceps pass in front of the pelvis as anterior crural, 
while those to the triceps pass behind the shoulder-girdle bound up in the radial.”* 
(2) In the fore limb there is a nerve which enters the forearm behind the external 
condyle, and takes part in the innervation of the extensor muscles situated below 
the elbow, viz., the “nerve to the anconeus.” This nerve is not represented in the 
hind limb in Hatteria, and it appears to be absent in the hind limbs of the great 
majority of Vertebrate animals. It is, however, probably represented by a nerve 
which was first described by RucE in Ornithorhynchus.t In this Ornithodelph, a 
branch of the anterior crural nerve passes through the fibres of the quadriceps, and 
makes its way downwards between the patella and the shovel-shaped process of the 
fibula. In this course it lies on the fibular condyle of the femur, and ends in 
tibialis anticus muscle. Among the objections which may be raised against the 
above view of the homology of this nerve, the following is perhaps the most promin- 
ent :—The “ nerve to the anconeus” passes over the radial condyle of the humerus, 
while Rucer’s nerve crosses the fibular condyle of the femur. Broadly speaking, 
however, the two nerves are serially homologous, as each may be described as @ 
nerve from the trunk which supplies the extensor muscles of the middle segment, 
prolonged to the extensors situated below the knee (or elbow) joints. 
Menobranchus. 
T have chosen to consider Hatteria first, because it presents such a combination 
of low and high characters, that, while presenting itself as a primitive type, it is not 
difficult to deal with in the light of the highest. We reach a lower grade in 
passing to the great perennibranchiate amphibia. : 
* J. of Anat. and Phys., Vol. VI., p. 53. 
+ Ruez (Morph. Jahrb. IV., 1878, p. 597) apparently does not regard this nerve as serially homologous 
to the “nerve of the anconeus.” I had no difficulty in verifying Ruee’s description in the Ornithorhynchus 
I dissected (see Fig. 7, a.cr.), but I have not met with this nerve in any other of the Vertebrate types I 
have examined. 
/ 
