520 VETERINARY PHYSIOLOGY 



The expansion of the lungs is very unequal, being 

 most marked at the anterior and lower margins and much 

 less marked posteriorly, especially round the roots. 



The ventilation of the air vesicles is thus much greater 

 in the former than in the latter parts, where, if the breathing 

 be shallow, the exchange of gases comes to depend more 

 largely upon diffusion from the air of the dead space 

 (p. 523). 



In these parts the proportion of carbon dioxide in the 

 vesicles will be greater and the proportion of oxygen less 

 than in the other parts of the lungs. 



When the diaphragm takes the chief part in inspiration 

 the breathing is said to be abdominal in type — when the 

 intercostals take the chief part it is said to be thoracic 



The diaphragm and the external intercostals are the 

 essential muscles of inspiration, but other muscles also 

 participate in the act. The nostrils dilate with each inspira- 

 tion. The nostrils expand due to the action of the 

 dilatores narium, which contract synchronously with the 

 other muscles of inspiration. Again, if the larynx be 

 examined, it will be found that the vocal cords slightly 

 diverge from one another during inspiration. This is 

 brought about by the action of the posterior crico-arytenoid 

 muscles (p. 551), and this movement is interfered with in 

 " roarers " (p. 5 53). 



Forced Inspiration — This comparatively small group of 

 muscles is sufficient to carry out the ordinary act of inspira- 

 tion. But, in certain conditions, inspiration becomes forced. 

 A forced inspiration may be made voluntarily ; often it is pro- 

 duced involuntarily. Every muscle which can act upon the 

 thorax to expand it is brought into play. The body and spinal 

 column are fixed. The head is thrown back and fixed by 

 the posterior spinal muscles. The forelimbs and shoulders are 

 fixed, and every muscle which can act from the fixed spine, 

 head and shoulder girdle upon the thorax is brought into play. 

 Normally, these act from the thorax upon the parts into which 

 they are inserted ; now they act from their insertions upon 

 their points of origin. The sterno-mastoids, sterno-thyroids, 

 and sterno-hyoids assist in expansion of the thorax. The serratus 



