672 A MANUAL OF VETERINARY PHYSIOLOGY 



from without. Constant evaporation is taking place from the foot, 

 and its loss is made good in the manner indicated. If the invisible 

 moisture which is always escaping from the foot be hindered in 

 its evaporation, the horn becomes sodden, crumbles away, and 

 resembles a grey cheese-like mass. This experiment can be readily 

 performed on the sole and foot-pad by accurately moulding to their 

 surface a sheet of gutta-percha and leaving it there. The practical 

 lesson is obvious : no impervious material should be applied to the 

 foot as a protection, or, if used, it should be ventilated. 



Use of the Moisture in Horn. — The amount of moisture contained 

 in horn is something considerable, and the rate at which it evaporates 

 is remarkable.* If parings of the foot-pad be enclosed in a bottle, 

 in a short time the interior will become bedewed with moisture. 

 The use of the moisture is to maintain the elasticity of the foot, 

 and prevent the part from becoming brittle. The agencies which 

 are at work to prevent the too rapid evaporation of moisture from 

 the wall are the thin, varnish-like layer which covers the hoof, and 

 the natural hardness of the external fibres of the wall. Horn con- 

 taining but little moisture is in an abnormal condition ; it is rigid 

 and brittle ; nails driven into the part cause it to crack, and the 

 elasticity, on which the natural shape and usefulness of the foot so 

 largely depends, becomes impaired, or even destroyed. A museum 

 specimen of a foot illustrates these facts very clearly ; in its dried 

 condition it is so brittle that, if dropped, it will frequently fracture 

 like a piece of glass ; but if this foot be placed in water for a few 

 days, it comes out as fresh and elastic as though it had just been 

 removed from the body. All that the horn has done is to imbibe 

 water, and the previously brittle substance now becomes yielding 

 and elastic. The entire physiology of the horse's foot is centred 

 around this question of the moisture contained in horn. The pres- 

 ence of moisture confers elasticity, and elasticity of the hoof prevents 

 its fracture under the pounding effects of concussion during work. 



Chemistry of Horn. — An analysis of the horn of the foot has given 

 us the following results :f 



Wall. Sole. Foot-pad. 



Water 24735 37'°65 4 2 '54 



Organic matter . . . . 74*740 62*600 57*27 



Salts 0-525 0-335 OI 9 



IOOOOO IOO'OOO ioo*oo 



The pad contains the largest amount of moisture, and the wall the 

 least. The salts are small in amount, and consist principally of 

 those of sodium, magnesium, iron, and silica, in the form of chlorides, 

 sulphates, and phosphates. Keratin, a substance which replaces 

 the protoplasm originally existing in the cells, is a protein-like body 

 found in hair, nails, and even, in a modified form, in the nervous 

 system ; it consists of carbon 51 "41, hydrogen 6-96, nitrogen 17*46, 

 oxygen 19*49, and sulphur 4*23, per cent. The sulphur is loosely 



* James Clark (' Observations on the Shoeing of Horses ') described the 

 moisture in the foot in 1782. He speaks of the insensible perspiration 

 exuding, and states that if a newly-pared piece of frog be held up to the 

 light of the sun, vapour may be seen arising from it. 



* ' Chemistry of the Hoof of the Horse, Veterinary" Journal, vol. xxv., 

 1887. 



