214 
Proceedings of the Royal Society of Edinburgh. [Sess. 
line. In the typical hand, it will be remembered, there is no adductor 
supplied to this digit. The musculi accessorii interossei 2 and 3 are there- 
fore the two heads of the flexor brevis unaltered in function. 
In the 4th digit the outer head of the flexor brevis has disappeared. 
It has acquired neither the position nor the function of a palmar inter- 
osseous muscle, as there is a well-developed contrahens producing adduction. 
The outer head of the flexor brevis muscle to the 4th digit is the only one 
of this series of muscles which is absent from the hand of this ape. The 
inner head is present as the musculus accessorius interosseus 4. 
The 5th digit has both heads of the typical muscle. The outer head is 
represented by a poorly developed palmar interosseous muscle, a fairly 
strong contrahens carrying out the function of adduction. The inner head 
is present as the flexor brevis minimi digiti. 
This analysis shows clearly the relations borne by the palmar interossei 
to the adductor or contrahentes muscles, and in a satisfactory manner 
establishes the true position of the musculi accessorii interossei. These 
muscles are no new development in this animal. They are merely deviations 
from the original type, the modification being the response to the special 
requirements of function. The habits of the animal demanding, as they do, 
a strengthening of the short flexors, have caused Nos. 2 and 3 of these 
muscles to become partially segmented into two bellies'; and in the same 
way increase in the function of the thumb has caused the segmentation of 
contrahens 1 into adductors transversus and obliquus pollicis. The same 
demand on the part of nature has caused the insertions of the muscles to 
migrate downwards to a point where they can act to better mechanical 
advantage. 
In mentioning the representation of the second or intermediate layer of 
muscles in the hand of man and this ape, it will be noticed that the flexor 
brevis minimi digiti is counted as belonging to this layer, although hitherto 
it has been the custom to regard this muscle as belonging to the first or 
adductor layer. (See Plate II. fig. 4.) 
The reason this muscle is usually grouped with the adductor layer is 
the position it holds with regard to the deep division of the ulnar nerve. 
The deep division of the ulnar nerve lies, in the typical manus, between the 
adductor and the flexor brevis layers of muscles. In the human hand the 
flexor brevis minimi digiti lies superficial to this nerve, and has, in con- 
sequence, been adjudicated to the former group of muscles. The change of 
function from an adductor to a flexor cannot be urged strongly against 
such a view, as we have already seen that many of the original muscles 
of the flexor layer adopt the functions of adductors when they become 
