The Foot. 369 



cartilages has light thrown on it by diseased processes. 

 When this elastic cartilage becomes converted into bone, 

 its functions are destroyed, and lameness occurs. I have 

 shown that by a simple operation relief from this lameness 

 may, in a large proportion of cases, be secured, and demon- 

 strated that by surgical interference it is possible to make 

 the horny foot larger, and thereby render it capable of 

 accommodating, without inconvenience to the animal, 

 lateral cartilages which have undergone the process of ossi- 

 fication, and thus increased in size. This operation is 

 based on physiological principles. 



4. The object of having elastic cartilages at this part of 

 the foot is to allow of expansion. 



Navicular Bursa. — The pedal bone presents a remarkably 

 small articulatory surface, much smaller than the surface 

 of the bone which rests on it. In order to increase this 

 articular surface a sesamoid bone is introduced, known as 

 the Navicular ; by this means the corona rests on an articu- 

 lation which anteriorly is rigid, but posteriorly is flexible. 

 Under this yielding sesamoid bone passes the perforans 

 tendon, and the weight on the bone is supported almost 

 solely by this tendon. I have shown* that the compression 

 to which the navicular bone is thus exposed by the weight 

 of the body from above, and the pressure of the perforans 

 from below, is the chief factor in the production of that 

 serious and common lameness, navicular disease. I do not 

 consider, as is generally believed, that the navicular bone 

 acts the part of a pulley. 



When the foot comes to the ground the weight of the 

 body comes first on to the flexible articulation in the pedal 

 joint, viz., the navicular bone and its supporting tendon, 

 the perforans ; the corona is then rotated in such a manner 

 that the w T eight of the body is transferred to the pedal bone, 

 and through this to the laroime. 



Wherever we look in the foot we find the same provi- 

 sion maintained for an elastic posterior and rigid anterior 

 foot. 



* Veterinary Journal. 1886, 'The Pathology of Navicular Disease.' 



24 



