520 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



column and accordingly increase the resistance, while heat, by diminish- 

 ing the cohesion of the liquid, will lessen the resistance. 



A similar state of affairs holds in tubes of varying calibre. In tubes 

 of unequal calibre, as already pointed out, the velocity of the current 

 will vary ; that is, it will be slower in the wide part of the tube and 

 more rapid in the narrower parts of the tube. And as the resistance 

 is greater in narrow tubes, the propelling force will diminish more 

 rapidly than in wide tubes. " When a small tube passes suddenly 

 into a tube of larger diameter there is a sudden increase of pressure at 

 the surface of junction, accompanied by a diminution in the speed of 

 movement through the wider tube. The molecules of which the fluid 

 consists cannot suddenly change the swift movement into a slower one, 

 and on account of their inertia the pressure exerted by them on one 

 another develops the increased force. On the other hand, the rapid 

 transition from a slow to a quick movement at a place where a wide tube 



FIG. 215. DIAGRAM ILLUSTRATING THE VARIATIONS OF PRESSURE IN AN 

 OUTFLOW-TUBE OF VARYING CALIBRE. (Rollet.) 



passes into a narrow one diminishes the pressure. The effect, however, 

 in a system of tubes of a series of wider parts is to diminish the total 

 resistance " (Robertson) (Fig. 215). 



Bending the tube adds a new resistance, the fluid pressing more 

 strongly on the convex than on the concave side of the bend, and, there- 

 fore, producing greater resistance to movement on the convex side. 

 Consequently, resistance is increased behind the bend and diminished 

 in front of it, with a consequent increase in the velocity of the current at 

 this point. When a tube through which liquid is passing divides into 

 two or more branches, still further resistance is added by not only in- 

 creasing the surface, but by the production of angles and bends. The 

 total calibre of the branches which originate from a single tube may be 

 either greater or less, and so the surface of contact between the walls of 

 the tube and the fluid are either increased or decreased. 



The most interesting case corresponds to that seen in the develop- 



