COMPOSITION OF BONE 175 



Cases have also been reported of the presence of hots (larvae of the 

 CEstrus equi) in the brain cavity of the horse, and also in the spinal canal 

 of a pony. 



Among the organs of special sense, the eye of the horse seems to 

 be the only one which is invaded by parasites. It is recorded that 

 Van Setten removed a pentastome from the right eye of a horse, and 

 in horses in India the presence of a nematode worm is e.xtremely common. 

 The parasite is easily removed by puncturing the cornea and allowing 

 the aqueous humour to escape, carrying with it the worm. 



A minute worm (Filaria i^aliyehratus) is occasionally found under the 

 eyelids of the horse, causing irritation, with swelling of the eyelids and 

 an abundant secretion of tears. 



14. ORGANS OF LOCOMOTION-BOJES 



COMPOSITION OF BONE 



All bones are made up of two parts: 1, an organic matrix; 2, mineral 

 matter or bone-ash. If the rib of a horse be macerated for a few days or 

 weeks in dilute hydrochloric acid, the mineral or earthy matter will be 

 dissolved out of it and the animal or organic matrix will remain behind. 

 In this condition it still retains its original form, but, having lost its 

 hardening constituents, it is now soft and flexible, and may be bent in 

 any direction like a piece of india-rubber, or even tied in a knot. 



If a second rib be placed in a bright clear fire and burnt, all the animal 

 matter is destroyed and driven oft', leaving the earthy substance behind as a 

 white brittle mass, and, as in the previous experiment, still retaining the 

 shape of the lione. 



The relative proportions of organic and inorganic matter entering into 

 the formation of bones vary at diflerent periods of life. In young animals 

 the former makes up nearly one-half of the whole, while in the adult it is 

 reduced to nearly one-third, the remaining two-thirds comprising earthy 

 or mineral substance. It is on account of the larger quantity of soft 

 oi-ganic matter they contain that the bones of young animals are so 

 much more yielding, and therefore less liable to break, than those of 

 older ones. The earthy substance of a bone consists of phosphate and 

 carbonate of lime in the proportions of 56 per cent of the former and 

 about 13 of the latter. The animal matrix, which is a kind of gelatine, 

 makes up the rest. 



