ABSORBENT SYSTEM 65 



and hepatic), heart, lungs, kidneys, brain, and spinal cord of other 

 mammals. Many of the veins within the cranium are included in 

 spaces formed by the separation of the laminae of the dura mater, 

 and do not admit of being dilated beyond a certain size ; these are 

 termed sinuses. The portal circulation in mammals is limited to 

 the liver, the portal vein being formed by the superior and inferior 

 mesenteric, the splenic, the gastroepiploic, and the pancreatic veins. 

 The kidney is supplied solely by arterial blood, and its veins empty 

 their contents only into the inferior cava. 



Lymphatic Fessels.—The absorbent or lymphatic system of vessels is 

 very fully developed in the Mammalia. Its ramifications extend 

 through all the soft tissues of the body, and convey a colourless 

 fluid called lymph, containing nucleated corpuscles, and also, 

 during the process of digestion, the chyle, a milky fluid taken up 

 by the lymphatics (here called lacteals) of the small intestine, and 

 pour them into the general vascular system, where they mix with 

 the venous blood. The lymphatic vessels of the hinder extremities, 

 as well as those from the intestinal canal, unite in the abdomen to 

 form the "thoracic duct," the hinder end or commencement of 

 which has a dilatation called the receptacvlum chyli. This duct, 

 which is of irregidar size and sometimes double, often dividing and 

 uniting again in its course, or even becoming plexiform, passes for- 

 wards close to the bodies of the thoracic vertebrae, and empties itself, 

 by an orifice guarded by a valve, into the great left brachio-cephalic 

 vein, having previously received the lymphatics from the thorax and 

 the left side of the head and left anterior extremity. The lymph- 

 atics from the right side of the head and right anterior limb usually 

 enter by a small distinct trunk into the corresponding part of the 

 right brachio-cephalic vein. The duct, and also the principal lymph- 

 atic vessels, are provided with valves. 



Lymphatic glands, rarely met with in the Sauropsida, are usually 

 present in mammals, both in the general and in the lacteal system ; 

 the latter being called " mesenteric glands." They are round or oval 

 masses, situated upon the course of the vessels, which break up in 

 them and assume a plexiform arrangement, and then reunite 

 as they emerge. No structures corresponding to the pulsating 

 " lymphatic hearts " of the lower vertebrates have been met with in 

 mammals. 



Ductless Glands. — Associated with the vascular and lymphatic 

 systems are certain bodies (the functions of which are not properly 

 understood), usually, on account of their general appearance, 

 grouped together under the name of "ductless glands." The 

 largest of these is the " spleen," which is single, and always 

 placed in mammals in relation to the fundus or left end of the 

 stomach, to which it is attached by a fold of peritoneum. It is dark- 

 coloured and spongy in substance, and has a depression or " hilus " 



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