128 PHYSIOLOGY OF FARM ANIMALS [CH. 



joint is thereby avoided or at any rate reduced, for although the 

 force of the body weight is partly transferred to the pedal bone 

 on which the navicular rests, the latter bone yields under pressure, 

 as does also the pedal bone. 



The chief support of the navicular is the perforans tendon 

 which passes beneath it, and so admits of the yielding articulation 

 just mentioned. There is a synovial apparatus^ attached which 

 serves the purpose of reducing the friction. Nevertheless the 

 navicular bone owing to the important part it plays, and its 

 exposure to compression, is very liable to disease, and any 

 inflammatory condition set up is likely to spread to the perforans 

 tendon and other adjoining parts. 



We may now consider more closely the structure and functions 

 of the hoof and the horn producing tissues, and the relation of 

 these parts to one another. 



The keratogenous membrane has upon its outer surface a large 

 number of leaves which constitute the sensitive laminae. These 

 leaves are longer at the toe than at the heel where they are short 

 and turned in to form the 'sensitive bars.' The keratogenous 

 membrane is devoted to leaf formation excepting on its inner 

 surface which encloses the pedal bone. 



The coronary substance extends all round the coronet. The 

 periople is secreted from that part of it which occupies the 

 cutigeral groove between the upper margin of the hoof and the 

 skin. On its lower edge the coronary substance fuses with the 

 fibres from the sensitive laminae, being continuous with the 

 keratogenous membrane as already stated. 



The insensitive foot or hoof covers the sensitive part com- 

 pletely. The foot is covered by the periople which is a varnishing 

 substance secreted in the coronary region where it is thickest. 

 It gradually gets thinner lower down the hoof. Its function is 

 to cement the junction of the skin to the hoof and also to control 

 evaporation from the hoof. 



The colour of the wall is black or buff, the pigment being 

 produced in the coronary substance. 



The wall is thicker at the toe than at the heel, since at the toe 



^ This is commonly called the navicular bursa, but its function appears to 

 be to lubricate the bone and parts concerned, and not merely to act as a cushion, 

 like the bursae over which the extensor pedis and other tendons pass. These 

 are little closed sacs and are so placed as to protect the parts from injury. 



