526 THE KIDNEYS. 



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 microscopip 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 

 number enter the cortical substance at every point of its surface, 

 and, after proceeding for scarcely half a line in its substance, 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 substance, 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 

 remarkable for the innumerable minute arteries into which they 

 divide previously to entering the capsule. The supra-renal vein 

 collecting the blood from the medullary venous plexus and receiving 

 the 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, which they approach by their upper extremities. 

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



* inches and a half in breadth, and somewhat more than one inch in 

 thickness ; and weighs between three and five ounces. The kidneys 

 are usually enclosed in a quantity of fat, they rest upon the dia- 

 phragm, upon the anterior lamella of the transversalis muscle, which 

 separates them from the quadratus lumborum, and upon the psoas 

 magnus. The right kidney is somewhat lower than the left, from 

 the position of the liver ; it is in relation by its anterior surface with 

 the liver and descending portion of the duodenum, which rest upon 

 it, and is covered in by the ascending colon and by its flexure. The 

 left kidney, higher than the right, is covered in front by the great 

 end of the stomach, by the spleen, descending colon with its flexure, 



* Muller's Archiv. 1836. 



