274 



ANIMAL BIOLOGY 



of the chest. The space between the two layers contains 

 a fluid which serves to prevent friction from the constant 

 movement of the chest in breathing. Pleurisy is a disease 

 due to the inflammation of the pleura. 



We have now to consider how the air goes in and out of 

 the ungs. The ribs enclosing the chest are capable of 

 more or less movement by means of various muscles which 

 are attached to them. By raising up the sternum or bone 

 to which the upper ribs are attached in front, and by 

 spreading the lower free ribs laterally the chest becomes 



enlarged, and, as the lungs ex- 

 pand at the same time, air tends 

 to rush in from without to fill 

 the extra space a process 

 which goes by the name of in- 

 spiration (literally breathing 

 in). At the lower side of the 

 cavity of the chest is a broad 

 muscular sheet, the diaphragm, 

 forming a complete septum 

 across the body. Usually the 

 diaphragm is arched upward in 

 the center, but when its mus- 

 cular fibers contract they cause this arch to be flattened 

 downward, thus further enlarging the cavity of the chest. 

 Relaxation of the diaphragm and contraction of the 

 muscles that draw in the ribs cause the air to be forced 

 out of the lungs a process which goes by the name of 

 expiration (literally breathing out). The alternate move- 

 ments of inspiration and expiration, while to a certain 

 extent under the control of our will, go on for the most 

 part quite unconsciously. Their rapidity varies with 

 age, being greater in young children, gradually becoming 

 slower as they grow older. It also varies with exercise 



FIG. 203. Diagram to illus- 

 trate the effect of depressing 

 the diaphragm on the air con- 

 tent of the lungs. 



