684 A MANUAL OF VETERINARY PHYSIOLOGY 



the leg. With every compression of the foot the water rises in 

 the manometer tubes, and falls during the period of no pressure, 

 a period corresponding in the living animal to the foot being 

 off the ground. 



We must accept it, therefore, as a proved fact that the venous 

 circulation is largely facilitated by the expansion and contraction 

 of the posterior part of the foot ; during expansion the blood 

 is being driven upwards, and during recoil the veins aspirate 

 the blood into their interior. Indeed, so perfect are these 

 mechanisms that there are no valves in the veins of the foot, 

 and none are found nearer than the middle of the pastern. To 

 assist the circulation, the large venous trunks at the postero- 

 lateral part of the foot are in close connection with the lateral 

 cartilages, and some of the vessels even pass through their 

 substance. 



A summary of the physiological features counteracting con- 

 cussion and facilitating circulation may be stated as follows : 



When the weight comes on to the foot, it is received by a 

 yielding foot-articulation, an elastic wall, an india-rubber-like 

 pad, and through these by the plantar cushion. The elastic 

 posterior wall is pressed outwards by the compressed pad and 

 plantar cushion, and it expands slightly from the ground surface 

 to the coronet. At the moment of expansion the bulbs of the 

 heel of the foot at the coronary edge sink under the body weight 

 and come nearer the ground, and as a result of this the anterior 

 coronary edge retracts. The pedal bone descends slightly 

 through its connection with the sensitive laminae, and presses the 

 sole down with it, while the wall of the foot slightly diminishes in 

 height owing to the compression to which it is exposed. Under 

 these conditions the blood-pressure in the veins of the foot rises, 

 and the vessels are emptied. When the weight is removed from 

 the foot the bloodvessels fill, the pad and posterior walls recoil, 

 the bulbs of the heel rise, and the foot becomes narrower from 

 side to side ; at the same time the anterior edge of the coronet 

 goes forward, and the pedal bone and sole ascend. 



The Nervous Supply of the foot has been mainly worked out 

 by Mettam.* Tactile sensibility has long been known to exist, 

 but the nature of the end organs concerned was unknown. He 

 has shown that these partake of the character of Pacinian cor- 

 puscles, arranged sometimes singly, sometimes in groups, and 

 with other positional modifications, which has enabled four 

 distinct methods of distribution in the plantar cushion to be 

 recognised. Pacinian corpuscles have also been recognised in 

 the vascular laminae and in the skin of the coronet and heel. 



* Op.cit. 



