20 STRUCTURE AND ECONOMY OF THE HORSE. 



and in small circumscribed bags. The fat tlms contained often 

 performs the important office of affording a cushion for parts 

 that would otherwise be exposed to injury ; thus we find that the 

 socket of the eye is abundantly furnished with this material. 



There arc two other important membranes which are exten- 

 sively found in animal bodies ; they are the serous and mucous 

 membranes. Whenever an internal part has an external open- 

 ing, we find that it is furnished with a mucous mevihrane, which 

 secretes mucus for its own protection ; but when the cavity has 

 no external opening, it is lined with a serous membrane, which 

 secretes a thin watery fluid to lubricate the parts, and pre- 

 serve them from injury by friction. Thus from the entrance 

 of the mouth and nostrils to the anus, throughout the whole in- 

 ternal surfiice of the bowels, a mucous membrane exists, by 

 Avhich the fluid is secreted, the nature of which gives a name to 

 the membrane, and which protects it from injury either by the 

 external air, or by the contents of the bowels. In like manner, 

 we find the bladder and urinary organs similarly lined. 



On the other hand, the cavity of the chest and the abdomen, 

 with their contents, as well as the internal surface of blood 

 vessels, are furnished with a serous membrane, which secretes a 

 watery vapour. 



These different membranes are very frequently the seat of 

 disease, and are subject to severe and dangerous inflammations. 



The admirable manner in which the various organs are packed 

 away in their proper cavities is worthy of particular notice. 

 The lungs and the heart are so adapted to the shape of the 

 chest, that there is at no time any vacant spot ; and the more 

 numerous contents of the bowels are so disposed, that while each 

 has sufficient freedom for the proper performance of its func- 

 tions, yet the whole are packed away with the most economical 

 care : — there is no void to be found. 



From this cursory view of the matter, it will be seen how 

 numerous and complicated are the structures which compose 

 the frame of an animal, and yet how admirably and harmoni- 

 ously they are combined together. — Ed.] 



CHAP. VI. 



ON THE SKELETON. 



[Bone, of which the fabric of animal bodies is composed, is a 

 material possessing firmness and stability, which adapts it for the 

 performance of the three-fold office of protecting the vital 

 organs, supporting the softer parts, and affording points of at- 

 tachment for the various muscles. It consists of animal matter 

 and earthy salts ; the former consisting of cartilage, gelatine. 



