160 UNCONSCIOUS NERVOUS OPERATIONS 



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which they are nourished; and nerve fibers from sympa- 

 thetic nerves form plexuses around the blood vessels 

 plexuses from which nerve fibers penetrate between the 

 muscle fibers of the muscular coat. 



222. The Capillaries are the minute branches of veins 

 and arteries which form a connection and means of com- 

 munication between the two sets of vessels. Their walls 

 are formed of an extremely thin membrane which is easily 

 ruptured by unusual pressure. It is in the capillaries 

 that occur all the changes which take place in the blood. 

 In the web of a frog's foot, under the microscope, the 

 movement of the blood in the capillaries may be seen. 



223. Osmosis is the term applied to the phenomena of 

 interchange between different fluids when in contact or 

 when separated by membranes or walls having minute 

 pores. By osmosis the blood inside the capillaries and the 

 tissue elements on the outside become intermingled, and 

 by the same means animal tissues live upon the lymph, 

 which is in turn replenished by the blood, while certain 

 elements pass from the protoplasmic cells of the tissues, 

 by means of the lymph, into the blood. It is not true, 

 however, that osmosis in living tissues is subject to the 

 same laws as that which takes place through dead animal 

 or vegetable membranes. 



224. Course of the Blood in the Body. In describing the 

 circulation of the blood it is customary to speak of the 

 general or systemic circulation, the pulmonary circulation, 

 and the portal circulation. These are convenient terms, 

 and are here retained for that reason. There is, however, 

 but one circulation, by which the blood leaving any one 

 part of the circulatory system returns to the same part 

 again. To do so it must pass through two sets of 

 capillaries and must be twice returned to the heart. The 



