82 BLUE BOYS. 



collect the blood from the lungs and transmit it to the 

 heart, are called veins instead of arteries. Having now 

 been submitted to the action of the lungs, it is taken up 

 by a multitude of minute vessels, which are continually 

 joining one another in their course, till they finally form 

 four large trunks, called the pulmonary veins. They 

 come two from each side, and enter at opposite sides of 

 the arterial auricle. Thence the blood is transmitted to 

 the arterial ventricle, the opening between which and the 

 auricle, is provided with a valve, whereof the construc- 

 tion and use are very similar to those in the venous heart. 

 The valve here, however, is only twice cleft, and from a 

 fancied resemblance to a mitre, is called the mitral valve. 

 The arterial ventricle is longer, stronger and thicker, 

 than the other, but in all other respects, it closely re- 

 sembles it. The contraction of the arterial ventricle 

 forces the blood into the aorta, the grand trunk of the 

 arterial system. This vessel is also furnished with semi- 

 lunar valves, in the same manner as the pulmonary ar- 

 tery. The venous auricle and ventricle are more capa- 

 cious, but possess less power of contraction, than those 

 of the other side, for having to send the blood only to 

 the lungs, which are but a short distance from them, an 

 equal degree of force with that exerted by the others 

 which throw it over the whole body, is obviously not 

 required. This completes our account of the heart, and 

 the course of the blood through its different cavities. 



Emily. Before we leave this part of the subject, 

 please to enlighten me on one point about which I am 

 uncertain. The cavities of the two hearts, you observed, 

 were perfectly distinct without the slightest communi- 

 cation. Now, though I do not doubt the fact, I am at a 

 loss to see the reason of it. 



Dr. B. If you were fully acquainted with the na- 

 ture of respiration, you could not be at a loss one mo- 

 ment, to perceive what mischievous effects such a com- 

 munication would produce. Venous blood, you know, 

 would be mixed with arterial, and blood which has not 



