Lastly, though the foot is extremely liable to suffer from the effects of extreme dryness or excessive humidity, especially with regard to the changes thus brought about in the nature of the horn, it is perforce exposed at all times to the varying condition of the roads upon which it must travel.  The intense dryness of summer and the constant damp of winter, each in their turn take part in the deteriorating influences at work upon it.

Though this subject might be indefinitely prolonged, this brief resume of the adverse circumstances to which the foot of the horse is exposed is sufficient to point out the extreme importance of its study to the veterinary surgeon.  So long as the horse is used as a beast of burden so long will this branch of veterinary surgery offer a wide and remunerative field of labour.

CHAPTER II

REGIONAL ANATOMY

Considered from a zoological standpoint, the foot of the horse will include all those parts from the knee and hock downwards.  For the purposes of this treatise, however, the word foot will be used in its more popular sense, and will refer solely to those portions of the digit contained within the hoof.  When, in this chapter on regional anatomy, or elsewhere, the descriptive matter or the illustrations exceed that limit, it will be with the object of observing the relationship between the parts we are concerned with and adjoining structures.

Taking the limit we have set, and enumerating the parts within the hoof from within outwards, we find them as follows: 

A. THE BONES.—­The lower portion of the second phalanx or os coronae; the third phalanx, os pedis, or coffin bone; and the navicular or shuttle bone.

B. THE LIGAMENTS.—­The ligaments binding the articulation.

C. THE TENDONS.—­The terminal portions of the extensor pedis and the flexor perforans.

D. THE ARTERIES.

E. THE VEINS.

F. THE NERVES.

G. THE COMPLEMENTARY APPARATUS OF THE OS PEDIS.

H. THE KERATOGENOUS MEMBRANE.

I. THE HOOF.

A. THE BONES.

THE SECOND PHALANX, OS CORONAE, OR SMALL PASTERN BONE.—­This belongs to the class of small bones, in that it possesses no medullary canal.  It is situated obliquely in the digit, running from above downwards and from behind to before, and articulating superiorly with the first phalanx or os suffraginis, and inferiorly with the third phalanx and the navicular bone.

[Illustration:  FIG. 1.—­THE BONES OF THE PHALANX. 1, The os suffraginis; 2, the os coronae; 3, the os pedis; 4, the navicular bone, hidden by the wing of the os pedis, is in articulation in the position indicated by the barbed line.]

[Illustration:  FIG. 2.—­SECOND PHALANX OR OS CORONAE (ANTERIOR VIEW). 1, Anterior surface; 2, superior articulatory surface; 3, inferior articulatory surface; 4, pits for ligamentous attachment.]