588 THE KIDNEYS. 



larger in the foetus than in the adult, and appear to perform some 

 office connected with embryonic life. The anatomy of these organs in 

 the foetus will be found in the succeeding Chapter. 



In structure they are composed of two substances, cortical and 

 medullary. The cortical substance is of a yellowish colour, and con- 

 sists of straight parallel fibres placed perpendicularly side by side. 

 The medullary substance is generally of a dark-brown colour, double 

 the quantity of the yellow substance, soft and spongy in texture, and 

 contains within its centre the trunk of a large vein, the vena supra- 

 renalis. It is the large size of this vein that gives to the fresh supra- 

 renal capsule the appearance of a central cavity: the dark-coloured 

 pulpy or fluid contents of the capsule, at a certain period after death, 

 are produced by softening of the medullary substance. Dr. Nagel * 

 has shown, by his injections and microscopic examinations, that the 

 appearance of straight fibres in the cortical substance is caused by the 

 direction of a plexus of capillary vessels. Of the numerous minute 

 arteries, supplying the supra-renal capsule, he says, the greater num- 

 ber enter the cortical substance at every point of its surface, and, 

 after proceeding for scarcely half a line, divide into a plexus of 

 straight capillary vessels. Some few of the small arteries pierce the 

 cortical layer, and give off several branches in the medullary sub- 

 stance, which proceed in different directions, and re-enter the cortical 

 layer to divide into a capillary plexus in a similar manner with the 

 preceding. From the capillary plexus, composing the cortical layer, 

 the blood is received by numerous small veins which form a venous 

 plexus in the medullary substance, and terminate at acute angles 

 in the large central vein. 



Vessels and Nerves. The supra-renal arteries are derived from the 

 aorta, from the renal, and from the phrenic arteries ; they are remark- 

 able for the innumerable minute twigs into which they divide previ- 

 ously to entering the capsule. The supra-renal vein collecting the 

 blood from the medullary venous plexus, and receiving several branches 

 which pierce the cortical layer, opens directly into the vena cava on 

 the right side, and into the renal vein on the left. 



The Lymphatics are large and very numerous ; they terminate in the 

 lumbar glands. The nerves are derived from the renal and from the 

 phrenic plexus. 



THE KIDNEYS. 



The kidneys, the secreting organs of the urine, are situated in the 

 lumbar regions behind the peritoneum, and on each side of the ver- 

 tebral column, whicli they approach by their upper extremities. Each 

 kidney is between four and five inches in length, about two inches 

 and a half in breadth, somewhat more than one inch in thickness, and 

 weighs between three and five ounces. The kidneys are usually en- 



* Muller's Archiv. 1836. 



