67 



When the blood passes from the capillaries into the veins, it 

 runs very slowly, but increases in speed as it reaches the heart. 

 The flow of blood in the veins is assisted by valves where it 

 has to asscend against the force of gravity. The valves are 

 numerous. They open toward the heart. They may be single, 

 double, or treble, according to locality. 



There are some important exceptions to this rule. Besides 

 other organs, the veins of the brain and the feet have no valves. 

 They are numerous in the legs above, but there are none below 

 the fetlocks. 



The passage of the blood through the veins is facilitated by 

 muscular and other pressure and the aid of the valves, which 

 are the most numerous, where they have the force of gravity 

 to overcome ; and yet 



§ 



THE VEINS OF THE FOOT HAVE NO VALVES. 



There must be an important reason for this exception. The 

 following, perhaps, may be advanced as a possible explanation 

 of this remarkable fact : In no other part of the system is 

 there so much liability to disturbing influences upon the circu- 

 latory apparatus as under the comparatively hard and unyield- 

 ing horny encasement of the sensitive part of the foot we denom- 

 inate the hoof. Excessive dryness or moisture will affect the 

 foot injuriously, the necessary consequences of which would 

 be an abnormal diminution or enlargement of the calibre of 

 the circulatory vessels. 



The inference to be drawn from the fact of the veins of the foot 

 having no valves has an important practical value. It is that under 

 all ordinary circumstances the veins of the foot are sufficiently 

 exposed to pressure by the movements of the animal to propel 

 the blood through them; and that the valves so necessary and 

 useful elsewhere in overcoming the force of gravity, would be 

 so liable to get out of order in this organ that they would 

 rather obstruct than facilitate the flow of blood through the 

 veins. The vessels would then became congested; enlarge- 

 ment would ensue, and this would create an internal pressure 



