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 lamin 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 gastro-epiploic, and the pancreatic veins. 
The kidney is supplied solely by arterial blood, and its veins empty 
their contents only into the inferior cava. 
Lymphatic Vessels.—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 receptaculum chyli. This duct, 
which is of irregular 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 vertebre, 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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