156 



VITAL FUNCTIONS. 



Human Lungs and Heart. Fig. 96, represents the 

 lungs, together with the heart and large arteries, and 



Fig. 96. 



diaphragm as they are situated in Man ; a, the heart ; 

 b b, the lungs ; c c, the diaphragm. The bases of the 

 lungs, it will be observed, rest upon the diaphragm, 

 with which they are always in close contact. The lungs 

 are distinguished into parts, called lobes, which are partial 

 divisions of their lower parts. The right side of the 

 chest being larger than the left, because the heart is 

 principally on the left side, the right lung has three 

 lobes, while the left has only two. The lungs are entire- 

 ly made up of air cells and blood vessels, intermingling 

 with each other in the closest manner, the two fluids 

 being divided from each other by the thinest mem- 

 branes. Thus as the blood passes through the lungs, it 

 is largely exposed to the influence of the air through 

 this membrane. 



Such is the extreme tenuity of these vessels that 

 Dr. Keil estimated the number of cells in the lungs to 

 be nearly 180 millions, and Dr. Hales, supposing each 

 air-cell at 1-1 00th part of an inch in diameter, compu- 

 ted that the whole surface in both lungs would be equal 

 to 20,000 square inches. It is upon this surface that 

 the venous blood is distributed by an equal infinity of 

 vessels, and by which means it is aerated, or changed 

 into arterial blood, which is then immediately sent to 

 renovate and vivify the whole system, as shown in the 



What are the relative situation of the lungs, heart, and diaphragm 1 

 What is the substance of the lungs composed of 1 What amount of sur- 

 face is it computed the air vessels of Jungs contain ? 



