158 CIRCULATORY SYSTEM. 



larger canal.* The course of rivers exemplifies this continual- 

 ly; while confined to narrow channels, they rush tumultuously 

 through them, but when they begin to expand themselves into 

 capacious basins, or to be divided into a multitude of smaller 

 channels, the current becomes slower, and in sorae cases imper- 

 ceptible, though the fact is clear, that an equal volume of wa- 

 ter is every where descending in the same period of time. 



The moisture conferred upon all parts by the circulation of 

 the blood, bears a sufficient analogy to the effects of irrigation 

 upon ground. The water may be conducted to the latter by a 

 canal, which is finally divided into an infinitude of streamlets, 

 which ramify every where, and from the porosity of their beds 

 percolate laterally, so that the whole field, even to its most 

 minute atom, is kept moistened. The streamlets, afterwards, 

 successively assemble again into a single canal, which bears off' 

 their superabundant water. From the nature of the particles of 

 blood, many of them are confined to their proper channels, and 

 can never pass off by percolation into the tissue, through which 

 the blood vessels ramify. This may be proved by the fact that 

 the red globules of blood have a diameter of from the two- 

 thousandth to the five-thousandth part of an inch, a size incon- 

 siderable as it is, yet too large to permit their flowing through 

 elementary fibres or atoms ; whereas serum, or the water of the 

 blood, may, from the extreme fineness of the particles, be ab- 

 sorbed by any tissue whatever; a circumstance entirely un- 

 questionable, both from daily observation, as, for example, in 

 soaking a piece of dried meat or a bone; ard from the reflec- 

 tion, that the air itself will hold a certain quantity of water in 

 solution. 



A question then arises whether the moisture of parts not sup- 

 plied with red globules of blood, comes in the living body ex- 

 clusively from infiltration or from a peculiar set of vessels called 



fourths of the rubbing surface. Independently of this circumstance, it is found 

 that there is a greater difference in the quantities of fluids passing through aper- 

 tures of different sizes than there is in the areas of the respective apertures. 

 This is accounted for by there being less friction between the particles of fluids 

 than there is between these particles and a solid; and, in the larger apertures, a 

 smaller proportion of the particles comes in contact with the solid. 



* It is computed that the blood moves 5233 times slower in the capillaries than 

 in the aorta. 



