RESPIRA TORY AND EXCRETOR Y SYSTEMS. 561 



The food which has been digested — rendered soluble and diffusible — 

 passes from the food-canal into the vascular system by two paths : — 



(a) All except the fatty material is absorbed by veins from the stomach 

 and intestine. These unite in a main trunk the portal vein. The 

 components of the portal vein are — the lieno-gastric from the 

 stomach (and also from the spleen), the duodenal from the duo- 

 denum (and also from the pancreas), the anterior mesenteric from 

 the intestine, the posterior mesenteric from the rectum. The 

 portal vein breaks up into branches into the liver, whence the 

 modified blood passes by hepatic veins into the inferior vena cava. 



{V) The fat passes through the intestinal villi into the lymphatic vessels, 

 , which combine to form a thoracic duct which runs forward, and 

 opens inlo the left subclavian vein at its junction with the left 

 external jugular. Here and there lie lymphatic glands. 



Respiratory System. — The lungs are pink, spongy bodies, 

 lying in the thorax, connected to the exterior by the bron- 

 chial tubes and the trachea, and to the heart by blood-vessels. 

 The pleural membrane which invests the surface of the lungs 

 is reflected on to them from the sides of the thoracic 

 cavity. When the lungs expand, the pleural cavity — between 

 the two folds of pleural membrane — is almost obliterated. 

 The thoracic cavity js separated from the abdominal cavity 

 by a partly muscular partition or diaphragm, which is sup- 

 plied by two phrenic nerves, arising from the fourth cervical 

 spinal nerves. By its contraction the diaphragm alters the 

 size of the thoracic cavity, and thus shares in the mechanism 

 of respiration. At the top of the trachea lies the complex 

 larynx, the seat of the voice in mammals. 



Anteriorly the larynx is supported on its sides and beneath by the 

 thyroid cartilage, behind this lies the ring-like cricoid, dorsally to the 

 cricoid are two small triangular arytenoids. 



Within the larynx there are stretched membranous bands — the vocal 

 cords. Beside the larynx is the paired thyroid gland. 



The Excretory System includes the blood-filtering organs 

 or kidneys, their ducts the ureters, and a reservoir or bladder, 

 into which these open. The permanent kidneys and their 

 ducts are formed from the metanephros and metanephric 

 ducts of the embryo. The bladder arises as a diverticulum 

 from the hind end of the gut, being in fact a remnant of the 

 intra-embryonic part of the allantois. It loses its connection 

 with the gut, and the ureters which originally opened into 

 the rectum follow the bladder and open into it. 



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