CIRCULATION OF THE BLOOD. 247 



ing it performs what is known as a circulation, the great fact 

 discovered by Harvey. Such a particle of blood passes from 

 the left ventricle into the aorta, and being propelled into some 

 branch of the aorta it reaches a corresponding radicle of the 

 system of the vencc cavce through the capillary system inter- 

 posed between the system of the aorta and the system of the 

 vence cavce. By the system of the vence cavce it reaches the 

 right auricle of the heart, and from that auricle passes into 

 the right ventricle, whence it is propelled into the pulmonary 

 artery, and being conveyed from some fine branch of the pul- 

 monary artery to the pulmonic capillary system, it reaches a 

 corresponding radicle of the pulmonary system of veins, and 

 thereby is carried back to the left ventricle, from which it was 

 supposed to set out. Such is the circulation of the blood. 

 Every particle of blood which sets out from the left ventricle, 

 provided it escapes disintegration, performs a similar course ; 

 yet in some instances its progress is more complicated. 



By the contraction of the cavities of the heart, say of the 

 left ventricle, is signified a sudden and powerful diminution of 

 its cavity, by the action of the muscular fibres which form its 

 walls. The effect of this contraction is to expel the blood. 

 But this ventricle has two apertures, one communicating with 

 the left auricle, the other leading into the aorta. If both 

 apertures were equally open at the moment of the contraction 

 of the ventricle, the blood would be driven out in two direc- 

 tions ; but while there is a valve commanding each aperture, 

 that which is in the mouth of the aorta, named the sigmoid or 

 semilunar valve, is so constructed as to yield readily when the 

 ventricle contracts ; on the other hand, the valve in two divi- 

 sions connected with the orifice leading from the auricle extends 

 into the ventricle, and is attached by numerous tendinous 

 threads to two short blunt muscular pillars (the musculi 

 papillares), which, contracting at the same moment as the 





