THE ELIMINATION OF METABOLIC PRODUCTS. 581 



The importance of the other glands, large and small, as organs for the 

 elimination of substances foreign to the organism and of the end-products of 

 metabolism is insignificant compared to that of the kidneys. Their anatom- 

 ical structure characterizes them for the exercise of their most important 

 function. In the first place we notice the peculiar nature of the blood- 

 vessels. The arteries form branches and side twigs; each of the afferent 

 vessels terminates in a globular bunch of capillaries, the glomerulus or Mai- 

 pighian tuft. The blood leaves the glomerulus through a so-called efferent 

 vessel, or Vas efferens, which also breaks up into a close capillary plexus 

 which surrounds the secreting tubes. From this network come venous 

 radicals, which empty into the veins of the kidneys, and through which 

 the blood, which has meanwhile been freed from the metabolic end-products 

 and other waste-material, leaves the kidneys. The termination of the 

 afferent vessels in the Malpighian tuft has awakened the most interest. 

 It is worth mentioning that these vessels are considerably narrower than 

 the corresponding efferents. 



The Malpighian tuft is within the so-called Bowman-Muller capsule. 

 This consists of a thin pouch consisting of epithelial cells, into which, as it 

 were, the tuft has been pushed. It represents the beginning of a uriniferous 

 tube. The latter are not simple drainage channels, but follow, first of all, 

 a tortuous path, the first convoluted tube, or tubulus contortus. Then the tube 

 narrows suddenly and describes a loop, reaching into the medulla, known 

 as Henle's loop. The tube then turns back towards the cortex, forming 

 an irregular convoluted tube, that of Schweigger-Seidel, which passes 

 through a narrower arch into the straight collecting tube. Several tubes 

 which have up to this point been entirely independent, empty into this 

 collecting tube. It terminates, together with other similar tubes, at the 

 surface of a papilla in the calyx of the kidney. We will state, moreover, 

 that the epithelium of these different parts of a uriniferous tube is not 

 uniform. We mention briefly these relations, in order to show that the 

 process of forming a secretion by the kidneys is by no means a very simple 

 process. There is some reason for the complicated construction of the 

 kidneys. In considering the function of the kidneys, we must hold close 

 to the anatomical relations, and attempt to explain the significance of the 

 differently organized parts of the uriniferous tubes. Before taking up the 

 question of the secretion of the urine, we will briefly mention, in connection 

 with the above brief description of the construction of the uriniferous 

 tubes, those researches which have been undertaken in the attempt to 

 decide what the function of each different part of the tube is. Right at 

 the start it may be stated, that, according to all we now know, the urine 

 is not eliminated from the blood in the form in which it eventually reaches 

 the calyx to be emptied into the bladder. There is probably an absorption 

 of substances, partly of water and partly of other substances, while it is in 



