82 ANIMAL BIOLOGY. [Part I. 



around the urinary tubules, which are lined with columnar or 

 polyhedral secreting cells. The course of the tubules is peculiar. 

 After winding about in the cortex (so that in a section the cut 

 ends, of numerous coils will be seen), each tubule passes down 

 into the medullary region ; but instead of passing at once to the 

 pelvis it turns back, making a sharp loop (Henle's loop), re-enters 

 the cortex, and there again becomes convoluted, after which 

 it again dives down into the medulla, and so reaches the pelvis. 

 The size of the tubule varies, being larger in the convoluted 

 parts of its course. This course is special to the mammalia. 

 In a stained section, therefore, the cortex is characterised by 

 the coiled tubules and glomeruli, while the medullary portion 

 shows the straighter parts of the tubules, and vascular bundles. 



It seems probable that from the glomeruli there passes into 

 the capsule little but water containing a certain amount of salts 

 in solution, the amount of fluid thus filtered from the blood 

 depending largely on the blood pressure. The urea is probably 

 secreted in the convoluted portions of the tubule surrounded 

 by the capillary plexus. 



In the frog there are glomeruli, capsules, and convoluted 

 tubules, but the latter have not the remarkable mammalian 

 course, and there is no distinction into cortical and medullary 

 regions. There is, it will be remembered (p. 28), a double blood 

 supply. That from the renal artery passes to the glomeruli, 

 that from the renal portal vein passes to the convoluted tubules. 

 And it has been experimentally proved that here, and not in the 

 glomeruli, is urea (injected into the blood) excreted. 



15. The Skin. A transverse section of the skin of the frog is 

 shown in Fig. 30, i. The external layer of the epidermis is com- 

 posed of flattened scales. A middle layer shows rounder 

 nucleated cells. A deeper layer shows larger columnar cells, 

 with their long axes vertical. Beneath this is the dermis, 

 composed largely of 'fibrous bands supporting nerves and blood- 

 vessels. In the uppermost part of the dermis are large pigment 

 cells. 



Imbedded in the dermis, with their narrow funnels passing- 



