394 UNGULATA 
or pedal bone. There is also a much smaller second extensor on 
the outer side of this in each limb, the lateral extensor of the 
phalanges. In the fore leg the tendon of this muscle (which corre- 
sponds with the extensor minimi digiti of man) receives a slip from 
that of the principal extensor, and is inserted into the first phalanx. 
In the hind leg (where it is the homologue apparently of the 
peroneus brevis of man) the tendon becomes blended with that of the 
large extensor. 
A very strong ligamentous band behind the metapodium, 
arising from near the upper extremity of its posterior surface, 
divides into two at its lower end, and each division, being first 
connected with one of the paired upper sesamoid bones, passes by 
the side of the first phalanx to join the extensor tendon of the 
phalanges. This is called in veterinary anatomy the “suspensory 
ligament of the sesamoids,” or of the “ fetlock ” (10 in Fig. 164) ; but 
its attachments and relations, as well as the occasional presence of 
muscular fibres in its substance, show that it is the homologue of 
the short flexor muscle of other mammals, curiously modified both 
in structure and function to suit the requirements of the Horse’s 
foot. Behind or superficial to this are placed the two strong tendons 
of the long flexor muscles, the most superficial, or flexor perforatus 
(8), dividing to allow the other to pass through, and then inserted 
into the middle phalanx. The flexor perforans (9) is as usual in- 
serted into the terminal phalanx. In the fore leg these muscles 
correspond with those similarly named in man. In the hind leg, 
the perforated tendon is a continuation of that of the plantaris, 
passing pulley-wise over the tuberosity of the caleaneum. The 
perforating tendon is derived from the muscle corresponding with 
the long flexor of man, and the smaller tendon of the oblique flexor 
(tibialis posticus of man) is united with it. 
The hoof of the Horse corresponds to the nail or claw of other 
mammals, but is so constructed as to form a complete and very 
solid case to the expanded termination of the toe, giving a firm 
basis of support formed of a nonsensitive substance, which is con- 
tinually renewed by the addition of material from within as its 
surface wears away by friction against the ground. The terminal 
phalanx of the toe is greatly enlarged and modified in form to sup- 
port this hoof, and the size of the internal framework of the foot is 
further increased by a pair of lateral fibro-cartilaginous masses 
attached on each side to the hinder edges of the bone, and by a 
fibro-cellular and adipose plantar cushion in the median part. 
These structures are all enclosed in the keratogenous membrane or 
“subcorneous integument,” a continuation of the ordinary derma of 
the limb, but extremely vascular, and having its superficial extent 
greatly increased by being developed into papille or lamine. From 
this the horny material which constitutes the hoof is exuded. A 
