78 STRUCTURE AXD ECOXOMY OF THE HORSE. 



be ordinavily imperceptible to the naked eye ; and they seem to 

 possess a peculiar sensibility for the absorption of chyle, which, 

 however, is urged onwards by means of capillary attraction.* 



Thus the beautiful process of digestion is carried on, whereby 

 the grass of the field is gradually converted into the various and 

 complicated structure of animal frames. In this process it is 

 easy to believe how the imperfection of one part may neutralise 

 the operation of another, and thereby materially impair the effect 

 of the whole. Thus, loss of condition, so often met with in the 

 horse, may arise either from deranged stomach or intestines, 

 from diseased secretions of the glands, or from obstruction or 

 disease of the absorbent vessels. — Ed.] 



CHAP. XVII. 



ON THE URINARY ORGANS. 



[The urine Is separated from the blood by means of the kidneys, 

 two glands of well-known shape, lying one on each side of the 

 spine, immediately under the lumbar vertebrae. Unlike the 

 liver, their lower surfaces only are covered by peritoneum ; and 

 they are kept in their situation partly by cellular membrane 

 which attaches them to the psoas muscles above, and to the 

 neighbouring parts ; partly by the fat in which they are im- 

 bedded ; but still more by lai-ge and important blood vessels, 

 which enter the gland at the notch in its centre. 



On cutting into the kidney, we find that the outer part is a 

 dark red, and the inner a lighter colour : the former is termed 

 the cortical, and the latter the medullary, part ; and the sub- 

 stance of the former runs into the latter. The kidneys are covered 

 by a membrane peculiar to itself. The blood enters the kidney 

 at its notch by means of the emulgent artery, which divides into 

 four or five branches, taking their course towards the cortical 

 part, where they end in minute vessels. These vessels, which are 

 exceedingly numerous, terminate in little globular bodies, by 

 which the urine is separated. The superfluous blood is returned 

 by the veins ; but the urine is conveyed by a particular set of 



* Modern researches in organic cliemistry have thrown considerable 

 light on the subject of digestion. It is now ascertained beyond doubt that 

 food consists of two characteristic elements, one being for the nourishment 

 of the body, the other for the support of combustion in tlie lungs. The 

 former contains nitrogen, and the latter does not. Thus flesh, grain, and 

 pulse contain nitrogen, and are capable of affording nourishment ; whilst 

 oil, fat, and starch are only able to supply carbon for the purpose of respira- 

 tion. Hay and corn contain both these elements in combination, and are thus 

 well adapted both for recruiting the wear and tear of the muscular structure, 

 and for keeping up the temperature of the body. — Ed. 



