THE FOOT 665 



the size of the coronary substance ; the wider it is, the larger the 

 area it affords to the horn-secreting membrane covering it. 



Horn Laminae. — On the inside of the wall of the hoof a number of 

 leaves of horn are found arranged side by side, running all round the 

 foot from heel to heel, and extending from coronet to ground surface. 

 It is easy to see that they correspond in size, direction, and length 

 to the vascular or sensitive laminae previously described, and, like 

 them, they possess secondary horn lamina or lamellcB (Fig. 225B). 

 These insensitive and sensitive laminae fit into each other by the 

 process of dovetailing, which results in extraordinary strength being 

 obtained. So powerful is the union that, in endeavouring to separate 

 them, the vascular laminae will often tear from the pedal bone rather 

 than rupture the dovetail. In this way the most intimate and perfect 

 union between the vascular and hoof wall is brought about, and, 

 in addition, other advantages are obtained which will be dealt with 

 shortly. The horn laminae, as their name implies, are composed of 

 horn, but the secondary laminae which invest them are composed of 

 cells which are a something between horn and epithelium — i.e., the 

 cells have not undergone a true horny conversion, but remain proto- 

 plasmic in nature ; this is recognised by the fact that they readily 

 stain with carmine, whereas horn does not. It will be remembered 

 that though the sensitive and insensitive laminae dovetail, yet they 

 are never in actual contact, for between them are the lamellae, both 

 sensitive and insensitive. The sensitive lamellae look towards the 

 horn wall, the insensitive lamellae point in the direction of the pedal 

 bone, so that the dovetailing of the lamellae is not affected at right 

 angles to the primary laminae (Figs. 2 25 a, 225B). The origin of the 

 horn laminae has been considered at p. 659. 



Horn laminae are found on the bars, for these, though situated 

 under the foot, are a part of the weight-bearing wall, and possess all 

 its essential structural elements. 



The Sole of every normal foot is concave (Figs. 229, 230), that of the 

 hind-feet being more concave than the fore. This concavity agrees 

 with the concavity of the solar surface of the pedal bone, which in 

 itself is ample evidence that the general surface of the sole is not 

 intended to bear weight, though the portion in contact with the wall 

 is a weight-bearing surface (Figs. 229, j; 230, 5). Soles vary in 

 thickness, some being rigid and firm, others thin and yielding ; the 

 sole cannot be too thick. Those shown in Figs. 220, 229, 230, 

 are excellent specimens of a good sole. The growth of the sole 

 is peculiar ; in exactly the same way as was noticed in the wall, 

 the papillae from the vascular sole fit into pin-point holes in the 

 horn-sole, and horn is developed around them. But here the re- 

 semblance ends ; while the horn of the wall is capable of growing 

 to almost any length, until, in fact, it curls like a ram's horn, the 

 horn of the sole can only grow a very short distance before the fibres 

 break off, and scales or flakes of horn are the result ; these either 

 fall out or are pulled out. In other words, the foot determines for 

 itself how thick the sole shall be, and, without any assistance, the 

 fibres break off when the proper thickness has been attained, and 

 allow the part to drop out. This shelling out of the sole, which 

 can be seen in Fig! 228, g, is advantageous in the shod foot, inasmuch 

 as the part, not being exposed to friction, cannot wear away. In 

 parts of the foot, such as the wall, which in the unshod foot are 

 exposed to friction, no breaking off of horn fibres occurs ; wear and 

 tear maintain the part at its proper length and thickness. The 



