THE URINARY ORGANS. 609 



present nothing but the pelvis and a certain number of clavatc hollow 

 protrusions continuous with it the calices. From each of the latter is 

 subsequently formed, by continued budding, a bundle of tubuli uriniferi, 

 each of which bundles ultimately constitutes a Malpighian pyramid and 

 the cortical substance appertaining to it ; whilst, at the same time, the 

 kidney expands into a corresponding number of lobes. The tubuli 

 uriniferi are at first solid, composed solely of cells, and without any 

 membrana propria. In the course of development the latter arises, 

 probably from a plasma afforded by the cells, and the cavity of the 

 canal is formed, in consequence, it may be supposed, of the connection 

 of a fluid between the cells ; simultaneously with which the tubules 

 begin to grow rapidly in length, and to become convoluted. The Mal- 

 pighian bodies are originally nothing more than solid, clavate, thick- 

 ened extremities of the rudimentary tubuli uriniferi. The interior 

 cells of these pyriform or rounded bodies, subsequently become capil- 

 laries, which are continuous at two points, with the vessels outside, 

 whilst the most exterior form the epithelium, which joins that of the 

 tubulus uriniferus, and, like it, is invested with a membrana propria, 

 which is, of course, deficient where the afferent and efferent vessels 

 enter and emerge, at which point it may be said to be perforated. In 

 the newborn child, according to Harting, the renal canals are three 

 times more slender than in the adult, whence, as the kidney of the 

 latter is only twice the size of that of the child, it is obvious that, at 

 any rate after birth, no tubules are formed. 



With respect to the physiological relations, I would merely offer the 

 following remarks. There can be no question that the peculiar vascular 

 conditions in the kidney, in accordance with which the blood circulates 

 in special coils projecting into the commencement of the tubuli urini- 

 feri, before it passes into the proper capillary plexus of the organ, are 

 most intimately connected with the secretion of a large proportion of 

 water in the urine. The hindrance to the flow of the blood gives rise 

 to a considerable lateral pressure in the glomeruli, and a large quantity 

 of blood-plasma is forced through the thin opposing membrane (the walls 

 of the capillaries and the epithelium). Since all the elements of the 

 plasma are not found in the urine, and those that are, present in it 

 totally different proportions to those in which they exist in the blood, it 

 is obvious that the membrane in question does not act simply as a filter, 

 but also, for reasons at present unknown, retains certain substances 

 (protein-compounds, fat), whilst it allows others (urea, &c.) to pass 

 through with peculiar facility. In this way there is formed in the com- 

 mencements of the tubuli uriniferi, probably a very diluted urine, which 

 afterwards, as it flows towards the pelvis of the kidney, reciprocally 

 acts and is acted upon by the blood circulating around the tubuli urini- 

 feri, receiving additional substances from it (perhaps more urea), but 



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