THE PULMONARY ARTERY. 81 



them up as far as their tendinous strings will permit. 

 This is sufficiently far for all the fringes to meet one 

 another, and thus form a perfectly impervious mem- 

 brane stretching across the opening. When the ventri- 

 cle dilates, the fringes are pulled down by the separa- 

 tion of the walls to which they are tied, and they hang 

 loose in the ventricle till they are again raised by its 

 contraction. Thus, you observe, they act the parts of 

 valves, and are called, tricuspid valves, from the pointed 

 appearance of the three main pieces. 



Emily. Ah, now I perceive why the blood which 

 fills the auricle does not run directly through it into the 

 ventricle. While the auricle is filling, the ventricle is 

 contracted and expelling its contents, so that the same 

 valves which prevent the blood from escaping into the 

 auricle, also prevent it from escaping into the ventricle 

 before the auricle is filled. Is not this correct ? 



Dr. B. Yes ; and now having a clear idea of it thus 

 far, you will comprehend the rest with very little diffi- 

 culty. The blood has now been pushed into the pul- 

 monary artery, but some contrivance is obviously re- 

 quired to prevent the blood from passing back again into 

 the ventricle while it is dilating. This object is also 

 effected by valves. Just at the mouth of the artery, are 

 three thin, strong membranes of a semi-lunar shape, 

 with their loose edges hanging free in the cavity of the 

 artery. The blood in the artery pressing in all direc- 

 tions, raises these valves till their sides are brought to- 

 gether, and thus the cavity of the artery is completely 

 closed up. Being prevented from passing in this direc- 

 tion, it flows on through the pulmonary artery, and is 

 finally distributed to every portion of the lungs. 



Emily. I cannot see with what propriety you call 

 this vessel an artery, for it certainly conveys venous 

 blood. 



Dr. B. It carries venous blood, indeed, but it has 

 the structure of the arteries, and its course is from, not 

 to, the heart. For the same reason, the vessels which 



