Human Physiology. 213 



which are placed in the course of the great arteries, and filter 

 in a short time the whole current of the circulation. Passing 

 into the kidneys by large arteries, the blood is distributed in 

 minute vessels, wound into convolutions as shown in Fig. 54, 

 mingled with organic nerve-fibres. The urine is drained from 

 the blood and carried into a central receptacle, from which it 

 passes by long tubes, the ureters, into the bladder, which these 

 tubes enter in such a slanting direction that no liquid can flow 

 backward perfect valves, such as are found in the heart and 

 veins, and wherever they are needed. People who eat flesh for 

 food pass much more urea, or solid matter of urine, than vege- 

 table feeders, because their blood contains the waste matter of 

 the flesh they eat, as well as of their own tissues, and some of 

 the severest forms of diseases of the kidneys may undoubtedly 

 be attributed to the inordinate eating of animal food. 



I have already mentioned in the chapter on the alimentary 

 system, that the whole course of the intestines was a vast col- 

 lection of glands of secretion and excretion. Some secrete 

 blood-forming matter from the food; others secrete waste mat- 

 ter from the blood, which passes on and forms the great bulk 

 of the faeces evacuation. This evacuation may become so 

 large as to produce rapid waste or the waste, beginning from 

 some cause in the system, may seek this mode of evacuation 

 as in diarrhoea and dysentery. Certain drugs excite these excre- 

 tions from the bowels, or are most readily got rid of in that 

 way. Purgative medicines are such as excite this action, some 

 by entering the circulation and finding this outlet, others by 

 simply stimulating or irritating the excretory glands of the 

 intestines. In either case the habitual or frequent use of them 

 is attended with great mischief, and they are seldom if ever 

 needed, for a proper diet and injections of cool or cold water, 

 and rubbing or kneading the bowels, are the only propel 

 remedies. 



The action of liver, spleen, kidneys, salivary glands, pancreas, 



