The percentage of ash and of phosphoric acid in the common foods of horses may be seen in the following table:
| Ash. | PO5 in the Ash. | PO5 in the entire food. | |
|---|---|---|---|
| Per cent. | Per cent. | Per cent. | |
| Wheat bran | 7.3 | 50 | 3.65 |
| Wheat grain | 3.0 | 46.38 | 1.3914 |
| Oats grain | 2.50 | 26.5 | 0.6625 |
| Barley grain | 3.10 | 39.9 | 1.2276 |
| Bean grain | 3.10 | 31.6 | 0.9864 |
| Pea grain | 2.75 | 34.8 | 0.957 |
| Tare grain | 3.00 | 36.2 | 1.086 |
| Indian corn grain | 1.5 | ||
| Rye grain | 1.6 | 39.9 | 1.0384 |
The source of the magnesia may be found to a large extent in the grains represented in the following table:
| Ash. | Mg. in Ash. | |
|---|---|---|
| Per cent. | Per cent. | |
| Oat, grain | 2.50 | 7.3 |
| Barley, grain | 3.10 | 8.5 |
| Rye, grain | 1.6 | 2.4 |
| Wheat, grain | 2.12 | 9.98 |
| Wheat, bran | 7.3 | 11.2 |
| Bean | 3.1 | 6.6 |
| Pea | 2.7 | 5.6 |
The amount of magnesia in each of these grains is amply sufficient to furnish the material for the constant growth of a calculus. Wheat bran is preëminent in the amount of its magnesia and therefore wheat bran has been charged with predisposing to calculi. In the perisperm as a whole, Fürstenberg found 1 per cent. of phosphate of magnesia, and in coarse bran not less than 2.5 per cent.
The ammonia which is essential to the precipitation of the phosphate of magnesia in the form of the compound salt (ammonio-magnesian) can be found wherever proteids are in process of septic fermentation. The slightest failure to peptonize every particle of such proteids, implies septic change and the evolution of ammonia, which on coming in contact with magnesia phosphate instantly precipitates the insoluble salt.
This fully agrees with the doctrine of the formation of urinary calculi through the agency of bacteria, since the ammonia is essentially a fermentation or bacterial product.
It may also be noted that the experiments of Rainey and Ord showed that in the presence of colloids (mucus, epithelium, pus, blood) the earthy salts are precipitated as minute globular bodies which by further accretions become calculi. In the absence of colloids the salts tend to precipitate in angular crystalline forms, so that the mulberry and coralline calculi may possibly have been precipitated in the absence of such bodies. From the solvent quality of ammonia, however, the contents may easily pass from a fermenting liquid containing colloids to a non-fermenting and noncolloid mixture.
The presence of a solid body which may act as a nucleus is an essential element, and the condition of the food or drink will often supply this. It has been noticed that army horses in the field, feeding from the ground and taking in sand and pebbles, are unusually liable to intestinal calculus. Horses which lick earth in connection with acidity of the stomach or other dyspepsia are specially subject to it. Horses watered from shallow streams with sandy bottoms, where they take in sand with the water, have been similarly affected. Millers’ horses, in the days of old process milling, suffered not alone because of the abundance of oat hairs in the feed but also on account of the grit from the millstones. Hay and other fodders that have lain on the ground and which contain earth and sand furnish other sources of such nuclei. Shingle nails and other small nails, pins, needles, coins, etc., which have mixed with the feed are common causes of trouble, and indeed any foreign body may become the centre and starting point of a calculus.
Catarrhal affections and other lesions of the mucosa, which furnish excess of mucus, beside pus, lymph and even blood as nuclei, are invoked as starting points of the calculi, but however true this may be in particular cases, irritation and catarrh appear to be much more frequently the result than the cause of the calculus.