RESPIRA TION 1 1 1 



Costal and Abdominal Respiration. — In the human subject two 

 types of respiration have been described, one costal, as in a 

 woman, and one abdominal, as in a man. It is now generally 

 admitted that the costal type in the woman is an artificial con- 

 dition, the result of compression. In the lower animals the 

 breathing is essentially abdominal. 



The Foetal Lung contains no air, and therefore sinks in water. 

 The first few inspiratory gasps at birth distend the alveoli, but 

 for some time the conditions present in the adult — viz., the 

 negative pressure in the pleural cavity, and the collapse of the 

 lungs on opening the chest — are not found in the very young 

 animal. They only occur when the cavity of the thorax is 

 larger than the lung in a state of collapse. In the fcetus the 

 lungs exactly fill the chest in the condition of expiration, and it 

 is not until the chest cavity grows, as it were, too large for the 

 lungs that a negative pressure in the thorax is produced. Later 

 on (p. 138) the cause of the first act of breathing will be ex- 

 plained. Thoracic development in young animals is very rapid ; 

 a foal will increase 4 cm. (ij inches) in circumference within 

 the first few hours after birth ; when this absolute increase in 

 chest capacity is established, a negative pressure in the pleural 

 cavity is obtained. 



Muscles of Respiration. — The action of the muscles of the 

 chest during respiration has been much disputed. The external 

 intercostals doubtless, from the direction taken by their fibres, 

 pull the ribs forward, and by so doing increase the transverse 

 diameter of the chest ; in this respect they are regarded as 

 inspiratory muscles. The internal intercostals, the fibres of 

 which run in an opposite direction to the external, draw the 

 ribs backwards and act as muscles of expiration ; and speaking 

 generally, we may say that those muscles which draw the ribs 

 forward are inspiratory, whilst those which draw them back 

 are expiratory. The following table shows the inspiratory and 

 expiratory muscles of the chest : 



Muscles of Inspiration. Muscles of Expiration. 



Diaphragm. Abdominal muscles. 



External intercostals. Internal intercostals. 



Serratus anticus. Transversalis costarum. 



Levatores costarum. Serratus posticus. 



Serratus magnus (during Triangularis sterni. 



laboured respiration). 

 Latissimus dorsi (during 



laboured respiration). 

 Scaleni (during laboured 



respiration). 



In some animals the ribs do very little work and the diaphragm 

 becomes ^the^chief respiratory muscle. In most quadrupeds the 



