Respiration. 81 



ing broadly, we may say that those muscles inserted into 

 the anterior surface of the ribs are inspiratory, whilst those 

 inserted into the posterior edge are expiratory, from which 

 we make the following table : 



Inspiratory Muscles of the Ribs. Expiratory Muscles of the Rib*. 

 External intercostals. Internal intercostal?. 



Serratus anticus. Transversalis costarum. 



Serratus magnus (during dim- Serratus posticus. 



cult respiration). 

 Levatores costarum. 



After the expiratory act there is a pause before the next 

 inspiration. During repose the process of expiration is 

 longer than that of inspiration, though the proportion 

 between the two is not invariable. During work, the value 

 of the inspiratory and expiratory acts are about equal. 



During inspiration a negative pressure exists in the 

 trachea, and during expiration a positive pressure. In the 

 pleural cavity a negative pressure exists during expiration 

 and inspiration, due to the attempt of the elastic lung to 

 collapse ; the value of this intra-thoracic pressure has been 

 ascertained for the sheep to be about one-eighth of an 

 inch of mercury, and during dyspnoea a negative pressure 

 of three-eighths of an inch (Landois and Stirling). 



We can recognise this negative pressure post-mortem by 

 the inrush of air produced by the collapse of the lungs 

 when the chest is opened. 



The number of respirations will vary with the class of 

 animal ; as a rule, the larger the animal the slower the 

 respiration : 



Rumination increases the respirations, and muscular exer- 

 tion in all animals at once causes them to rise. In my own 

 experiments on respiration this has been most marked ; even 

 walking a horse will nearly treble the number of respira- 

 tions, and unless the fastest pace has been employed the 



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