48 AMERICAN FARMER'S HORSE BOOK. 



The right ventricle coraraiinicates with the right auricle bj 

 an orifice provided with a valve. From it arises the pul- 

 monary artery, through which the venous blood is forced to 

 the lungs by the contraction of this ventricle. After being 

 purified in the lungs, the blood is carried back to the left 

 auricle of the heart by the four pulmonary veins, (eight in 

 number when they leave the lungs,) which thus carry arterial 

 blood. From the left auricle the blood passes through the 

 mitral valve into the left ventricle, whose powerful walls 

 contract and force it out into the aorta, the great arterial 

 trunk, from which diverge, by successive subdivisions, all 

 the other arteries in the body. 



We have seen that there is no communication through the 

 muscular partition separating the right and left sides of the 

 heart. This mode of structure gives much increased power 

 to the contraction of the heart, which is simultaneous upon 

 both sides. 



The blood, then, is forced through the arteries by successive 

 impulsions, which are caused by contractions of the heart. 

 When an artery of considerable size comes near the surface, 

 these successive impulsions may be distinctly seen through 

 the skin and coats of the artery,- or they may be felt by lay- 

 ing the finger upon them. This is the simple explanation of 

 the pulse in one's wrist. Similar pulses are found in other 

 parts of the body, but none so plain and distinct. We say 

 the pulse is slow when the heart acts sluggishly; or fast 

 when, from disease or excitement, its action is heightened. 

 The beats of the pulse simply indicate so many impulsions 

 of the blood from the heart. 



THE PORES. 



These have been partially described, in the extract from 

 Youatt introduced in the section treating of the skin. They 

 are the extremities of the capillaries. Physiologists classify 

 them as of two kinds — absorbents and exhalants. The former 

 take up and carry into the circulation portions of the differ- 

 ent fluids and gases with which they come in contact on the 



