366 THE HORSE. 



lime. They also contain an animal and extractive matter, to which 

 the brown color of the calculus is owing. They also contain mu- 

 riates of soda, and various alkaline salts derived from the intestinal 

 juices. The animal matter resembles that of all other concretions, 

 and separates in concentric laminae when the calculus is dissolved 

 in an acid. In more impure varieties, grains of sand, portions of 

 hay, straw, &c., are frequently found imbedded in the calculus, 

 and there is one specimen in the museum which contains an entire 

 layer of vegetable hairs. Mr. Carter remarks that ' his impression 

 is that the calculus is made up of bran' (chemically speaking). 

 He is not far wrong, for we read in the College catalogue, ' Most 

 authorities agree that these calculi are formed from phosphate of 

 magnesia, contained in wheat, oats., Jiay^ &c., and this opinion de- 

 rives confirmation from the circumstance that they occur most fre- 

 quently in millers' and brewers' horses, which are fed upon grains, 

 bran, and substances known to contain a much larger proportion 

 of magnesian salts than other vegetable matters/ Mr. Carter has 

 detected minute portions of wheat, oats, and hay in the calculus, 

 which therefore may be said to consist of two substances, viz., the 

 vegetable and the mineral. So much, then, for the composition 

 of the calculus; now for its mechanical structure. Most decidedly 

 it may be compared to an onion, layer being packed over layer, so 

 as in section to present a ringed appearance. We may also liken 

 it to other objects. It has lately struck me to examine the struc- 

 ture of a common cricket-ball, which combines hardness, light- 

 ness, and elasticity in such an admirable way. Upon making a 

 section, I found the cricket-ball to be composed of layers, one over 

 the other, round a central nucleus. The layers are composed of 

 leather, alternated with a vegetable fibre, the nucleus being a bit 

 of cork. The calculus in the horse is formed in a similar way. 

 The nucleus in Mr. Carter's specimen is a bit of flint; in a capital 

 instance I have in my own collection, of a common shot, about 

 No. 5 size, which has been crushed by the horse's teeth, and sub- 

 sequently swallowed; in another instance, of a chair nail of brass; 

 in another of a single oat-seed ; in another of a minute bit of 

 cinder, and so on, as it seems to be absolutely necessary that these 

 calculi should have a commencement a starting-point. Where 

 is the school-boy who can make a gigantic snowball without be- 

 ginning with a small lump of snow or a stone, as a nucleus upon 

 which he builds all the rest ? 



" Mr. Carter seems to wonder at the weight of the specimen, 5 

 Ibs. ; this is by no means a large size; in the museum of the 

 Royal College of Surgeons we have a very fine collection of cal- 

 culi, the largest, taken from the intestines of a horse, weighs no 

 less than 17 Ibs., and is about the size and shape of an ordinary 

 skittle-ball. In the case where this is contained he will see many 



