SECT. I90.] DEVELOPMENT OF THE KIDNEYS. 417 



of club-shaped cavities connected with it, and representing the 

 calyces. From each of these cavities tufts of urinary tubules are 

 next formed by the continued formation of processes, and each of 

 these tufts is ultimately transformed into a Malpighian pyramid, 

 with its appertaining cortical substance; the kidney, at the same 

 time, grows into a corresponding number of large lobes. The 

 urinary tubules are at first solid, composed only of cells, straight, 

 and without a membrana propria. In the course of development, 

 this latter membrane makes its appearance, probably from a plasma 

 secreted by the cells, and the cavities of the tubules become de- 

 veloped, probably from the collection of a fluid between the cells ; 

 nt the same time, the tubules begin to grow quickly in length, and 

 to become tortuous. The Malpighian corpuscles are originally 

 nothing but solid, clavate, thickened terminations of the rudiments 

 of the tubules. At a later period, the inner cells of these pyriform 

 or roundish bodies are converted into capillaries, Avhich are con- 

 nected at two places with the external vessels; whilst the more 

 external are transformed into an epithelium, which becomes con- 

 tinuous with that of the urinary tubules, and becomes surrounded, 

 in like manner, with a membrana propria. This membrane is, of 

 course, wanting at the parts where the vessels enter and pass out. 

 According to Horting, the tubules are three times narrower in the 

 newly-born infant than in the adult, and, as the kidney of the 

 latter is only double the size of that of the child, it is evident 

 from this that no tubules are developed subsequently to birth. 



On the physiology of the kidney, I shall only make the following 

 remarks. It cannot be called in question, that the peculiar ar- 

 rangements of the vessels in the kidney, the flow of blood through 

 coils projecting into the commencement of the urinary tubules, 

 before it passes into the proper capillary network of the organ, is 

 most intimately connected with the excretion of large quantities 

 of water by the urine. Through the impediments to the flow of 

 the blood in the glomeruli, a considerable lateral pressure is pro- 

 duced, and a large quantity of blood-plasma must be forced through 

 the thin opposing membrane, that is, through the coat of the 

 capillaries and the epithelium. But as all the constituents of the 

 plasma are not found in urine, and as those which are present are 

 found in very different proportions in the urine and in the blood, 

 it is obvious that the membrane in question does not act simply 

 like a filter, but, by means still unknown to us, retains certain 

 matters, and allows others to pass through with great facility. 

 Of the former kind of substances, protein matters and fat, of the 



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