210 Proceedings of the Royal Society of Edinburgh. [Sess. 
1st Dorsal Interosseus ( Abductor Indicis). 
Origin. — (1) From the base of the 1st metacarpal on its inner side ; 
(2) from the whole length of the outer side of the metacarpal of the index, 
but rather more on its anterior than on its posterior surface. 
Insertion. — Into the outer side of the proximal phalanx, and slightly 
into the dorsal extensor expansion. 
Structure. — The muscle is penniform, fleshy at its origin but tendinous 
towards its insertion. It is separated below from the first palmar inter- 
osseous by the first musculus accessorius interosseus. 
2nd, 3rd, and Iftk Dorsal Interossei. 
These correspond closely to the same muscles in man, and need no 
detailed description. 
Origin. — From the sides of the two metacarpals between which they 
lie, additional fibres being received from the metacarpal bone of the digit 
on which they act. 
Insertion. — Nos. 2 and 3 are inserted into the outer and inner sides 
respectively, near the base of the proximal phalanx of the middle digit, and 
into corresponding margins of the dorsal extensor expansion. No. 4 is 
inserted into the inner side of the base of the proximal phalanx of the 
4th digit, and into the edge of the dorsal extensor expansion. 
Nerve Supply. — From the deep division of the ulnar; the fibres in the 
case of the outer three coming from the median in the forearm, while the 
fourth is supplied by fibres which travel all the way in the ulnar nerve. 
Structure. — The muscles are all bipenniform, and end in small tendons. 
They are situated dorsal to all other muscles, but can easily be seen from 
the front. 
Before discussing peculiarities of muscular development, it is always 
well to consider the particular functions which the muscles are called upon 
to perform. Function has a profound influence on the outward form of all 
organs ; modification of shape and position is frequently an adaptation to 
special requirements. In no organ perhaps is this better exemplified than 
in muscle, which readily and rapidly responds to any persistent demand of 
nature. The function of the hand of the gibbon has therefore to be called 
to mind if the muscular arrangement is to be understood. 
The upper limb of the animal is of extraordinary length. The humerus 
alone is longer by 5 cm. than the head and trunk, measured from the 
vertex to the ischial callosities. When it is borne in mind that the 
