Intestinal calculi are dense masses of earthy matter derived from the salts of the food, and by their close texture assume the form of stones - a term by which they are commonly known. They usually present a gray or yellowish appearance, and a smooth surface more or less polished. In form they are either round or angular (figs. 107, 108).

This difference depends upon the number present. Where they occur in a solitary state they are rounded, but where two or more exist in contact with each other, their constant movement one against the other during the action of the bowels wears away the surfaces of contact, thereby producing flattened facets or concavities and prominent angles. As a rule they are found in a solitary condition, but it is by no means rare to meet with several together, and the writer has removed as many as sixty from the large bowels of a horse. Where the bowel contains a number, their rubbing action upon each other prevents them from developing to any considerable size, but we have known a single one to reach as much as 65 lbs. weight.

The origin of intestinal calculi is clearly shown by dividing them through the middle with a saw, when it will be found that the centre is occupied by a nucleus of foreign matter, such as a nail, a button, a piece of wire, or particles of grit, etc. etc. If the cut surface of the stone be examined, and especially after having been roughly polished, it will be seen to be made up of a number of layers of earthy matter placed one outside the other in widening circles. Radiating lines are also seen extending from the centre to the circumference, marking out the calculus into triangular blocks. The outer surface is usually smooth, and in this respect they contrast with similar formations known as "concretions". Calculi are almost invariably found in the large bowel, where their formation and retention are favoured by the peculiar pouched arrangement of the organ. Their composition is expressed in the following analysis by Giradin: -

Group of Faceted Calculi.

Fig. 107. - Group of Faceted Calculi.

Rounded Calculus.

Fig. 108. - Rounded Calculus.

Ammonio-phosphate of magnesia.......

48.00

Calcic phosphate.........

19.00

Water........

14.00

Animal matter..........

•80

Soluble salts, etc..........

6.60

Extractive matters..........

4 00

Fatty matter...........

7.00

Loss.........

.

•60

100.00

It is generally believed by veterinarians that horses in the possession of millers and bakers are more frequently the subjects of these formations than those employed by other persons. Various reasons have been rendered for this exceptional liability to stone formation. While some attribute it to largely feeding on bran and other offal, others find an explanation in the presence of grit in the sweepings of the mill and bakehouse which the horses of these traders are said to receive. Before special machinery came into use for the removal of foreign matters from hay and chaff a considerable mortality from this cause prevailed in the studs of railway companies, contractors, and other large proprietors.

Where this cleansing process is not adopted obstruction of the bowels by calculi is still a common occurrence.