196 OF ELECTIVE FILTEATION. 



bly connected ; and, moreover, an enlarged view of the operation of cell 

 life seems to indicate that the general action of those organisms is to 

 produce a formative result, the grouping of amorphous into organized 

 material, and the elaboration of that material into more complicated and 

 higher forms. But many of the most important constituents of the va- 

 rious secretions are indisputably things which are on the downward ca- 

 reer, fast passing to the inorganic state. Many of them, as presented in 

 the bile or in the urine, run through a series of spontaneous changes, 

 which end in the appearance of truly inorganic bodies. For the fabrica- 

 tion of such substances, half inorganic themselves, it is scarcely to be 

 thought that cell life should be necessary ; and these, with many other 

 such considerations, recall the observation I made a few pages back, that 

 the more profoundly we study the composition and constitution of se- 

 creted fluids, and the more accurately we understand the function of se- 

 cretion itself, the less are we disposed to invoke the agency of cell life, 

 and to rely the more on the ordinary mechanical act of strainage. 



That the different secreting surfaces exercise an elective elimination on 

 Elective filtra- materials existing in the blood, some permitting the escape 

 tlon * of one, and some of another ingredient more readily, may be 



demonstrated from their action on saline substances purposely introduced 

 into the blood. Thus the iodide of potassium was detected by Bernard 

 in the saliva, pancreatic juice, and the tears in less than one minute, 

 but in the urine and bile not until after an hour. The ferrocyanide of 

 potassium could be recognized in the urine in seven minutes, but not at 

 all. in the saliva. In like manner, cane-sugar and grape-sugar appear in 

 the secretions of the kidneys and liver, but not in those of the pancreas 

 and salivary glands. The lactate of iron, injected into the veins, fur- 

 nishes no iron to the saliva, but both iodine and iron can be recognized 

 in Jliat secretion after the administration of the iodide of iron. 



Upon the whole, we may therefore conclude that very many substances 

 are strained from the blood in which they naturally occur by membranes 

 and glands, which, from the circumstance that they are of various con- 

 struction and possess a different physical nature, are better adapted, some 

 for the removal of one, and some for the removal of another compound. 



Among secreting surfaces the mucous membranes are usually enumer- 

 Of mucous ated. Strictly speaking, however, they are scarcely so much 

 mfdtheir^secre- secretm g surfaces as the seat of numberless secreting organ- 

 tion. isms. They line the interior of the digestive, respiratory, 



urinary, and generative apparatuses, and are characterized by extreme vas- 

 cularity. In structure they consist of several different layers or regions, 

 the undermost being submucous cellular tissue, upon which is spread the 

 proper mucous membrane, containing connective and elastic tissue, which 

 affords a nidus for blood-vessels and nerves. Upon this is the basement 



