I 9 6 ANIMAL BIOLOGY [Part I. 



and block the way to blood attempting to flow in the opposite 

 direction. 



Into this system of tubes the blood is forced at each con- 

 traction (systole) of the muscular walls of the ventricles of the 

 heart. The arteries are swollen with blood, the heart-pressure 

 being to some extent stored up in the stretched fibres of the 

 elastic coats of these vessels. After the systole of the ventricles 

 is over, the blood is passed on through the arteries and arterioles 

 to the capillaries. And now the muscular contractility of the 

 walls of the arterioles comes into play. Nerves from the sym- 

 pathetic system are distributed to their muscular walls. Sup- 

 pose, then, that an organ is quiescent and in need of but little 

 nourishment. Through the influence of the nerve-fibres the 

 muscular walls of the arterioles contract ; less blood is supplied, 

 and the blood-pressure in the capillaries falls. But now suppose 

 that the organ is active and in need of much blood and nourish- 

 ment. The walls of the arterioles are caused to relax ; a full 

 tide of blood flows to the part ; blood-pressure increases in the 

 capillaries ; there is more exusion of plasma through the epi- 

 thelioid walls, and nutrition increases. 



When the blood reaches the capillaries, unless the arterioles 

 be very widely distended, the wave-like flow, noticeable when a 

 great artery is cut, has ceased and given place to a continuous 

 flow. The elasticity of the walls of the elongated tubes, and the 

 distribution of the pressure over a large area of capillaries, has 

 effected this. The pulse that we feel in an artery of the human 

 wrist is the shock of systole propagated through the vessel, and 

 must not be confounded with the onward flow of the blood. 



It only remains to be noticed, in these preliminary remarks, 

 that the partial vacuum in the thorax of the rabbit, and sundry 

 other minor causes, may slightly add to or slightly impede the 

 flow of the blood. 



The Cod Fish. The heart of the cod (Fig. 59) consists of a 

 thin-walled sinus venosus (s. v.\ which receives blood from the 

 hepatic veins (he. v.) and the dudus Cuvieri (d. c.), of which only 

 the right vein receives an inferior jugular factor (?'. ;. v.). Below 



