CH. VIIl] FROGS, TOADS, AND NEWTS 275 



Excretory System. The excretion of the nitrogenous 

 waste substances produced throughout the body is per- 

 formed by the two kidneys. These are reddish-brown 

 organs of an elongate-semilunar shape, lying parallel to 

 the vertebrse close to the dorsal wall of the abdomen, and 

 outside the peritoneal lining. They receive blood both 

 from the renal arteries and from the renal portal veins. 

 The renal veins conduct the blood from them into the 

 posterior vena cava. The kidney substance itself is com- 

 posed of very numerous uriniferous tubes, which excrete 

 .water and urea from the blood. Each tube commences 

 blindly within the ventral portion of the kidney in a 

 globular dilatation the Malpighian capsule. The cavity 

 of the capsule is largely occupied by a cluster of blood 

 vessels the glomerulus derived from a twig in the renal 

 artery. Strictly speaking the glomerulus is external to 

 the capsule ; it appears to be inside it in consequence of 

 one-half of the capsule being thrust inward inside the 

 other, in the same way as one-half of a soft rubber ball 

 may be tucked inside the other by pressure with the 

 finger. The course of the individual tubes, though very 

 tortuous, is constant. The epithelium of the first portion 

 of the tube is ciliated, then follows a glandular portion 

 whose cells contain a yellow pigment and are said to bear 

 short cilia (Tornier 1 ) : the next part is densely ciliated and 

 passes into a wider portion, the protoplasm of whose cells 

 shows a rod-like structure. This part opens at right angles 

 into a collecting tube. The collecting tubes run across 

 the dorsal surface of the kidney into the ureter, which, 

 1 Arch. f. mikrosk. Anat. xxvu. 



182 



