er layer of non-fibrous horn. The designations "wall" and
"crust" are often used indiscriminately.

The _frog_ is the triangular horny cushion which is in the centre of the
ground surface of the hoof, and which, by its elasticity and strength,
acts as a buffer in saving the structures inside the hoof from the
injurious effects of concussion.

_The cleft of the frog_ is the division in the middle line of the frog.
In healthy feet, it consists of only a slight depression. In a disease,
called "thrush," of the sensitive part which secretes the frog, the
cleft forms a deep, damp and foul-smelling fissure, and the frog becomes
more or less shrivelled up. The frog similar to the skin of the palms of
our hands, requires frequent pressure to make it thick and strong. The
horn of the hoof is merely a modification of the cuticle (scarf skin).

The _bars_ of the hoof are the portions of the wall of the hoof which
are turned inwards at the heels, and run more or less parallel to the
sides of the frog. The _sole_ is that portion of the ground surface of
the foot which is included between the wall, bars and frog.

The _pastern_ (9) is the short column of bones (two in number) which
lies between the fetlock and hoof.

The _fetlock_ (8) is the prominent joint which is just above the hoof.

The _cannon bone_ (7) is the bone that extends from the fetlock to the
_knee_ (6), which, in the horse, corresponds to our wrist.

The _back tendons_ or _back sinews_ (_M_) form the more or less round
tendinous cord which is at the back of the leg, from the knee (or hock)
to the fetlock. These tendons, which are two in number, usually appear
in the form of one cord; but in horses which have a very fine skin and
"clean legs," we may see that one of them is placed behind the other.
The term "clean legs" signifies that the limbs are not only sound, but
are also free from any fulness, which would more or less obscure the
contour of the bones, tendons and ligaments. _Muscles_ are the lean of
meat, and their ends are connected to bones by means of _tendons_, which
consist of hard, fibrous and inelastic material. The _ligaments_ of the
limbs are composed of the same material (white connective tissue) as
tendons, and serve to connect bones together, without the intervention
of muscle. The horse has practically no muscles below his knees and
hocks.

[Illustration: Fig. 147.--External parts of horse.]

The _suspensory ligament_ is the fibrous cord whi