256 PHYSIOLOGY AT THE FARM. 



especially made up of nitrogen. The structure of the kidney 

 is somewhat complex. In mammals generally the kidneys are 

 two in number, one on each side of the vertebral column. The 

 surface of the kidney is smooth, and the colour is deep-red. 

 The kidneys are imbedded in fat. They are covered by a pro- 

 per fibrous coat ; the same coat passes into the interior of the 

 kidney by the fissure named the hilum, andjlnes the sides of 

 what is called the sinus, then surrounds the blood-vessels, giv- 

 ing sheaths to the chief of these, and accompanying them into 

 the substance of the gland. The solid part of the kidney con- 

 sists of two substances, the cortical and the medullary. The 

 cortical is exterior, immediately beneath the fibrous capsule, 

 and forms the superficial part of the organ to the depth in the 

 human kidney of about two lines. The cortical substance also 

 sends prolongations inwards towards the sinus, between which 

 prolongations the medullary substance is found. The medul- 

 lary substance is collected into a series of conical masses, the 

 bases of which are directed towards the surface of the kidney, 

 whilst their apices are turned towards the sinus. Their number 

 varies from eight to eighteen. The greater part of each pyra- 

 mid is imbedded in the cortical substance, but each apex is 

 free, and projects into the sinus under the name of mammillary 

 process. The cortical portion forms about two-thirds of the 

 substance of the kidney, while the medullary portion makes up 

 the remaining fourth part of the organ. Each medullary cone 

 is originally a separate lobe, and this lobular structure remains 

 throughout life in many animals. The entire substance of the 

 kidney, whether cortical or medullary, consists of uriniferous 

 tubes, blood-vessels, lymphatics, and nerves connected in some 

 parts with a fine areolar tissue. 



The excretory duct of the kidney is the ureter, which extends 

 from the inner concave surface of the kidney to reach the side 

 and base of the bladder in the pelvic cavity. At its connection 



