THE EXCBETORY ORGAN'S. 185 



being allowed to rest, although the clot is softer than that of the 

 blood. 



The thoracic duct extends from the loins to the neck, and its 

 course is along the spine. It is the principal trunk of the absorb- 

 ent system, and, as has been explained, is the connecting link be- 

 tween the digestive organs and the circulatory system, as the pul- 

 monary artery and vein is the connecting link between the circu- 

 latory and respiratory system. 



2Jie spleenis another organ which is very important, as being the 

 seat of a rather obscurely understood disease, known as splenic 

 apoplexy. It consists of a spongy mass of tissue of a mottled blue 

 or purplish gray color. It is suspended near the great curvature 

 of the stomach, but of its functions nothing is precisely known. 

 It is supposed to act as a reservoir of blood for the portal vein ; it 

 is also supposed to destroy the red globules of the blood, as it 

 has been discovered to contain blood globules in a state of decom- 

 position. It is, however, known that, in the course of researches 

 to discover the uses of this gland, animals from which it has been 

 removed have recovered from the operation, and have continued 

 to live in apparent good health. The fact of its engorgement 

 with blood in the disease of ruminants known as splenic fever or 

 apoplexy, and its increase of volume in certain bilious disorders, 

 would tend to show that its functions are in some way closely 

 connected with the circulation, and perhaps with the digestive 

 processes and nutrition. 



The Urinary or Excretory Organs. The urine is separated from 

 the arterial blood by the kidneys. These organs, with the liver 

 and the lungs, are employed in the purification of the blood. The 

 liver separates compounds abounding in hydrogen, the lungs those 

 which abound in carbon, and the kidneys those abounding in ni- 

 trogen. The nitrogen eliminated through the kidneys exists in 

 the form of urea, a crystalline substance which readily decomposes 

 and gives off its nitrogen in the form of ammonia. There are two 

 kidneys, one each side of the spinal column. The kidneys are 

 attached firmly to the loins ; in the sheep they are shaped like a 

 bean, and are imbedded in fat. They perform a double office, or 

 two separate functions, one being to discharge from the blood any 

 excess of water that may accumulate in it ; the other being to rid 

 the blood of excess of saline matter and the products that result 

 from the waste of the tissues. The blood enters the kidneys by 

 arteries, and the urine, separated as by a filter, through a very com- 

 plex system of capillaries, flows into two white ducts termed ure- 

 ters, which pass it onwards to the bladder. The urine of the 



