334 DISSECTION OF THE THORAX. 



the pericardium, the dissector should be careful not to injure the parts 

 contained in the interpleural space in front of the spine. 



PHYSICAL CHARACTERS OF THE LUNG. The surface of the lung is 

 smooth and shining, and is invested by the pleura. Through the serous 

 covering the mass of the lung may be seen to be divided by septa, into 

 small irregularly-sized pieces or lobules. On looking closely at it, espe- 

 cially at a thin margin, minute cells will be perceived in it. 



The tint, of the lung varies with age. In infancy the color is a pale 

 red; but in the adult the texture becomes grayish, and presents here and 

 there dark gray spots or lines of pigment, whose shade deepens with in- 

 creasing age, and becomes even black in old people. After death, the 

 color of the posterior border may be bluish-black from the accumulation of 

 blood. 



To the touch the lung is soft and yielding, and on a section the pulmo- 

 nary substance appears porous and spongy ; but the lung which is deprived 

 of air by pressure has a tough leathery feel. Slight pressure with the 

 thumb and finger drives the air from the containing cells through the pul- 

 monary structure, and produces the noise known as crepitation. If the 

 lung contains serum, a frothy red fluid will run out when it is cut. 



The texture of the lung is very elastic ; this elasticity causing the organ 

 to diminish greatly when the thorax is opened, and to expel air that may 

 be blown into it. 



The specific gravity of the lung varies with the conditions of dilatation 

 and collapse, or of infiltration with fluid. When the pulmonary substance 

 is free from fluid, and filled with air, it floats in water; but when it is 

 quite deprived of air it is slightly heavier than water, and sinks in that 

 fluid. The weight of the lung is influenced greatly by the quantity of 

 other material contained in its texture ; ordinarily it ranges from eighteen 

 to twenty-one ounces, and the right lung is about two ounces heavier than 

 the left. In the male the lungs are larger, and slightly heavier than in 

 the female. 



Dissection. By tracing the large branches of the bronchi, and the 

 bloodvessels and nerves into the lung, the mode of branching of the air 

 tubes will be apparent; and by inflating a part of the lung, the cellular 

 structure may be seen. But the arrangement of the small air cells about 

 their tube, and the disposition of the different vessels, cannot be ascer- 

 tained without fine injections and a microscope. 



STRUCTURE OF THE LUNG. The spongy pulmonary tissue consists of 

 minute recesses or cells, in which the smallest branches of the air tube 

 terminate; and the mass of the lung is formed by the collection of those 

 cells into small groups or lobules, and by the aggregation of the lobules 

 into larger masses or lobes. Each lobule is distinct from its fellows, and 

 is furnished with its air tube and nerves, and with its set of vessels con- 

 cerned in the function and nutrition. 



The parts of the lung are united by a serous covering, prolonged con- 

 tinuously over the surface ; and by a subserous layer of areolar tissue 

 which penetrates into the anterior, subdividing it into pieces. These 

 several parts are examined more in detail below. 



Serous and subserous coverings. The casing derived from the pleura 

 is thin and transparent, and forms an entire capsule for the lung, except 

 at the root where the vessels enter. The subserous areolar layer contains 

 fibres of elastic tissue, and not only covers the surface, but extends in- 



