EXCRETION AND THE EXCRETORY ORGANS 529 



circumstances the secretion of urine is suppressed. If the lower 

 end of the cut cord be stimulated the vessels all over the Body of 

 the animal contract, and blood-pressure everywhere becomes very 

 high. But the kidney vessels being constricted with the rest allow 

 very little blood to enter the glomeruli in spite of the high aortic 

 pressure, and little or no urine is secreted. If, however, the vaso- 

 constrictor nerves of the kidney be cut before the stimulation of 

 the cord, we get a dilation of the kidney vessels with a constric- 

 tion of vessels elsewhere, and abundant blood flows through the 

 glomeruli under high pressure: the whole kidney swells and abun- 

 dant urine is formed. When the skin vessels contract on exposure 

 to cold, more blood flows through internal organs, the kidneys 

 included, and the blood-pressure in these is if anything increased, 

 the expansion of internal arteries not at the most more than 

 counterbalancing the constriction of the cutaneous. Hence the 

 greater secretion of urine in cold weather. 



Diuretics. Various substances, caffein, digitalis, urea, salts, 

 and even water, stimulate the kidney to increased activity. Sub- 

 stances which have this effect are known as diuretics. It appears 

 that these act for the most part by stimulating the secreting cells 

 of the tubules to greater activity. 



The Skin, which covers the whole exterior of the Body, con- 

 sists everywhere of two distinct layers; an outer, the cuticle or 

 epidermis, and a deeper, the dermis, cutis vera, or corium. A blister 

 is due to the accumulation of liquid between these two layers. The 

 hairs and nails are excessively developed parts of the epidermis. 



The Epidermis, Fig. 142, consists of cells, arranged in many 

 layers, and united by a small amount of cementing substance. 

 The deepest layer, d, is composed of elongated or columnar cells, 

 set on with their long axes perpendicular to the corium beneath. 

 To it succeed several layers of roundish cells, b, the deepest of 

 which, prickle-cells, are covered by minute processes (not indicated 

 in the figure) which do not interlock but join end to end so as to 

 leave narrow spaces between the cells; in more external layers the 

 cells become more and more flattened in a plane parallel to the 

 surface. The outermost epidermic stratum is composed of many 

 layers of extremely flattened cells from which the nuclei (conspic- 

 uous in the deeper layers) have disappeared. These superficial 

 cells are dead and are constantly being shed from the surface of 



