630 



SPECIAL PHYSIOLOGY. 



cations. It serves important uses in the economy, enabling the whole 

 system to be maintained, for a time, upon itself, and, by the absorp- 

 tion of fatty matter stored up in the adipose tissues, supporting the 

 respiratory function, even in the absence of food. In the removal 

 and casting out of diseased products, or dead parts, it also exercises a 

 useful and conservative office. 



In conclusion, it may be repeated that, in addition to these impor- 

 tant uses, the function of absorption generally, ministers to the nutri- 

 tive function, by the conveyance into the circulating system, not only 

 of the materials of the food, but also of the residual part of the plasma 

 of the blood, not immediately employed in the nutrition of the tissues 

 amongst which it is poured out; and, lastly, that it assists in the 

 elaboration of those essential organized elements of the blood, its 

 white and its red corpuscles. 



The Absorbent System, and Absorption in Animals. 



A lymphatic and lacteal apparatus exists only in the Vertebrate Subking- 

 dom. In all cases, the finest vessels commence by blind extremities, and the 

 absorbent trunks empty themselves ultimately into the veins, forming, as it 

 were, a closed system superadded to, or constituting an offset from, the blood 

 system, with which, in the lower Vertebrata, it communicates at a great 

 number of points, not only in the neck, but also in the abdomen and pelvis. 



In Mammalia generally, as in Man, well-developed lymphatic glands are 

 found ; in the Carnivora, owing probably to the shortness of the intestine, the 

 mesenteric glands are so closely aggregated, as to appear like a large conglom- 

 erate gland. In Birds, lymphatic glands are also found, especially in the 

 forepart of the body, but they are less perfectly developed, and, in other parts, 

 are replaced by elaborate plexuses of lymphatic vessels ; in accordance with 

 the general lateral symmetry of these animals, there are two thoracic ducts, 

 each with its receptaculum chyli ; in certain tfirds, as in the goose, dilatations 

 of the pelvic lymphatics are met with, the coats of which are provided with 

 unstriped muscular fibres, which do not contract periodically or rhythmically ; 

 the lymphatics of the hinder part of the body communicate very frequently 

 with the veins. In Reptiles, the lymphatic glands are absent, but their place 

 is apparently supplied by the great size and abundance of the absorbents 

 themselves, and by numerous plexuses of closely-packed vessels ; the valves 

 are either imperfect, or are found only in the larger trunks ; communications 

 with the veins exist in the lower limbs. In this Class, as well as in the Am- 

 phibia and Fishes, there occur, connected with the lymphatic s} r stem, those 

 remarkable rhythmically contractile sacs, known as lymphatic hearts; they 

 have been found in the neck of certain Ophidia, and in the pelvis of the turtle 

 and crocodile. In the Amphibia, the lymphatics are relatively large, but few 

 in number ; neither valves nor lymphatic glands exist ; the lymphatic hearts, 

 usually four in number, have walls composed of striated muscular fibres : in 

 the frog, two of these hearts are situated, one on each side of the neck, oppo- 

 site to the third cervical vertebra, and two posteriorly in the pelvic region. It 

 is in Fishes that the absorbents are fewest in number ; they are delicate 

 transparent vessels, destitute of valves, excepting at the points of entrance 

 into the veins, which are here very frequent : the lacteals appear almost des- 

 titute of distinct walls. In the tail of the eel, and in many fishes, behind the 

 cranium, outside the jugular veins, there are found pairs of lymphatic hearts, 

 or dilatations of a similar nature to the true lymphatic hearts of the Am- 

 phibia. No lymphatics have been observed in the amphioxus. 



The chyle varies in color and opacity in different animals ; thus, it is very 

 milky-looking in the carnivorous, but almost colorless in the herbivorous 

 Mammalia ; it is also more transparent in the cold- than in the warm-blooded 

 Vertebrata. 



