PART II. 



NORMAL AND PATHOLOGICAL HIS- 

 TOLOGY OF NUTRIENT FLUIDS. 



I. GENERAL VIEW OF THE CIRCULATION. 



Next in importance to living matter itself is the nutrient 

 material for its physical support. The manner in which this 

 is distributed to the various tissues first claims our attention 

 in a general outline, reserving further details to future study. 



1. Absorption ot aliment is first in the train of vital actions. 

 In plants and in unicellular animals this takes place by simple 

 imbibition, but in higher animals the presence of a stomach, 

 or special reservoir for food, is -universal. The alimentary 

 canal is lined with epithelial cells, which in the stomach pre- 

 side over nutrition, exercising a power of selection so that only 

 certain materials may pass inwards, or be absorbed, while the 

 passage of other substances is resisted. Thus wooara, so 

 fatal by inoculation, is harmless in the stomach, not from 

 modification of the gastric juice, since mixture with gastric or 

 intestinal fluids does not destroy its power. The living cells 

 of the alimentary canal can by no means be regarded as inert 

 or passive, but perform important functions in absorption. 



2. Circulating vessels. In invertebrates nutriment is con- 

 veyed from the alimentary canal to the various parts of the 

 body by a general system of blood vessels, but in vertebrates 

 an intermediary set of vessels exists between the nutritive 

 material and the circulating blood. This intermediate appa- 

 ratus is of two kinds, like two sets of roots from a single 

 trunk. They are the lacteal s and the lymphatics. In mam- 

 malia these vessels are abundantly furnished with what are 

 termed glands. The vessels for conveyance of "nutrient fluids 

 are, therefore, the arteries, veins, capillaries, lymphatics, and 



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