306 INTERNAL ANATOMY OF IIOESE. 



branes, called the jplmrm. Part of these membranes surround 

 the walls of the thorax, part the lungs themselves. There is 

 one pleura on each side. Although the two pleural sacs are 

 apparently distinct, they are no doubt connected by a minute 

 opening in the mediastinum. 



The lungs are spongy in texture and are two in number, 

 each lying in a separate serous sac. The right lung is larger 

 than the left — the heart being lodged in an excavation between 

 the two. The lungs are very vascular. Blood is sent to them 

 by the piilmonary artery, which carries impure blood to the 

 lungs from the heart : here this impure blood is oxygenated, 

 and sent back to the heart by the pulmonary veins. These are 

 functional vessels : there are also nutrient vessels, the lym- 

 phatics, which ndt only lie around the outer portions of the 

 lungs, but also penetrate the internal lobules. 



The organs of excretion, or the urinary organs, consist of two 

 kidneys, ureters, bladder, and urethral canal. 



The kidneys eliminate the nitrogenous waste from the blood 

 along with water resulting from the exercise of the vital 

 functions. The kidneys are two in number, and are situated in 

 the sub-lumbar region of the body, lying against the great psoas 

 muscles. They are enclosed more or less completely in an 

 envelope of cellular fat. Support is also given to the kidneys 

 by the pressure of the organs in the abdominal cavity, and 

 again by the peritoneum, which underlies the urinary organs. 

 The right kidney (fig. 160, B) is in front of the left, lying 

 beneath the two last ribs, the left just beyond the last pair. 

 They also vary in shape and size, the right one being heart- 

 shaped, and weighing about twenty-seven ounces j the left one 

 (A) is kidney-bean-shaped, and about two ounces less. On one 

 side is a deep notch called the hilus. The outside of the kidney 

 is smooth and red in the horse ; very different is the kidney of 

 the ox, which is lobulated. .From the hilus arises the ureter or 

 tube which conveys the urine from the kidney to the bladder. 

 The ureter (a and h), which originates in the so-called pelvis 



