350 PHYSIOLOGY. 



rhomboids, extensor muscles of vertebral column; also some of the 

 laryngeal, palate and facial muscles. 



Expiration. Expiration, when it is effected with the aid of 

 muscular powers, has as its causative agents the internal intercostals, 

 the triangular sterni, the two oblique and transverse muscles of the 

 abdomen, posterior inferior serratus and quadratus lumborum. It is 

 in complex expiration as crying, coughing, singing, expectoration, 

 sneezing, etc. that the preceding muscles enter into contraction. The 

 abdominal muscles are the most powerful in the above-named group. 

 In general, it may be said that any and all muscles concerned in the 

 depression of the ribs belong to the expiratory set of muscles. 



On the contrary, ordinary expiration can be affected by the mere 

 relaxation of those factors concerned in the production of inspiration. 

 During this relaxation the thoracic and abdominal walls, by reason 

 of their elasticity, compress the air-distended lungs, and by so doing 

 compel expiration. The lung-tissue itself helps to the extent of its 

 own elasticity. The expenditure of that power and energy necessary 

 to produce inspiration now becomes the expiratory exponent. Dur- 

 ing ordinary and tranquil breathing this elastic recoil of the stretched 

 components is amply sufficient to expel the air from the lungs. Thus 

 no muscular energy is required to perform expiration. 



A normal lung is never able to contract to its fullest ability, 

 since it is always distended to some extent by reason of its cohesive 

 attraction with the interior of the chest-walls, as well as because of 

 the presence of a certain proportion of air within the vesicles which 

 exerts an expansive pressure. 



It is interesting to note that, though the expiratory muscles be 

 more numerous and powerful than the inspiratory ones, it is because 

 the former are intended especially for complex expiration; that is to 

 say, violent actions, since ordinary expiration is able to be effected by 

 the mere elasticity of the parts. During expiration the lungs, which 

 were dilated, return upon themselves, so that they let out a quantity 

 of air nearly corresponding to that which entered at first. The lungs, 

 w r hich are seen to be entirely passive during inspiration, can partici- 

 pate actively in expiration, particularly in such complex acts as ex- 

 pectoration, coughing, etc, 



MODES OF RESPIRATION. 



There are various modes of respiration in man and in mammals 

 which are usually classed under three principal types. In the abdom- 

 inal type, characteristic among children, the ribs remain motionless 





