THE ELIMINATION OF METABOLIC PRODUCTS. 585 



of urine, just as we have met with such substances in the study of other 

 glands. To be sure, it has been shown that a great many of these sub- 

 stances are capable of exerting only indirectly an influence upon the 

 kidney-cells, by affecting the flow of the blood through the kidneys on 

 the way to the vascular innervation. 1 For other substances, the result 

 cannot be traced to this cause. 



In order to prevent misunderstandings, we will at once state that purely 

 physical processes undoubtedly do play an important part here as in the 

 case of all absorption and secretion processes in the animal organism. 

 It is perfectly possible that a pure filtration process may act in conjunction 

 with other processes in the formation of urine. At the same time it would 

 be wrong to assume that the existence of a nitration process is proved. 

 We can only infer that it takes place, and are at once compelled to make 

 the auxiliary hypothesis of the reabsorption by the epithelium of the 

 uriniferous tubes. Another possibility seems to us as far more probable, 

 namely, that the Malpighian tuft is not the sole place where the constituents 

 of the urine are secreted. We have already seen that the efferent vessels, 

 after emerging from the tuft, again break up into a capillary network, 

 which surrounds the secreting tubes in the cortex. This behavior of the 

 blood-vessels must surely be of some significance in the formation of the 

 urine. It is possible that a back-absorption takes place here, but it is also 

 conceivable that the epithelium of the uriniferous tube is only able to 

 withdraw definite substances from the blood, concentrates them, and 

 finally sends them on at different periods towards the lumen of the tubes. 

 First of all, we must remember that the Bowman's capsule possesses numer- 

 ous finely branching nerve-fibers, which originate in the vasomotor nerves. 

 The blood-capillaries, also, are abundantly supplied with nervous plexuses. 

 Furthermore, it has been found that nervous branches exist which supply 

 the uriniferous tubes; and in fact we see separate plexuses of non- 

 medullated fibres, arising especially from the Tubuli contorti. Such 

 nerve-endings have also been observed for the epithelium of the Bow- 

 man's capsule, for the straight tube and also for the collecting tubes. 

 The fact that the vessels of the kidneys, and especially those of the 

 capillaries of the cortex have such an extensive innervation, suffices to 

 account for the sensitiveness of the renal vessels to all sorts of different 

 influences. More and more it has become evident that a great number 

 of those substances to which has been ascribed a specific action upon 

 the parenchyma of the kidneys, only influence the formation of urine by 

 accelerating or retarding the flow of the blood. The intimate dependence 

 of the secretion of the kidneys on the blood-supply may have been the 

 chief reason for assigning to the kidneys a position different from that of 

 the other organs of the body. 



1 Cf. O. Loewi: Arch, exper. Path. Pharm. 53, 15, 33, and 49 (1905). 



