Chap, xv.] EESPIRATOET ORGANS IF ANIMAL KINGDOM. 239 



more and more from the digestive tube ; it acquires aerian canals 

 more and more long and ramified, kept constantly open by a 

 cartilaginous skeleton, and supporting at one or more points of 

 their passage the organs of the voice. Over all its surface, the 

 aerian tree, trachea, and bronchi, is clad with vibratile epithelium ; 

 and this is the case for all surfaces in the whole animal kingdom. 

 The vibratile vestment is lacking only on the surface of the cells 

 or pvdmonary alveoli. 



Muscles, more or less numerous, impress on the costal frame 



Flo. 34, 

 Capillary network of the pulmonary vesicles. 



which covers the lungs and keeps them dilated alternative move- 

 ments of inspiration and expiration; but the diaphragmatic 

 partition which so powerfully aids inspiration in man and the 

 mammifers is completely lacking in reptiles, and is only in a 

 very rudimentary state in birds. 



We are compelled, that we may not indulge in a parergon, to 

 renounce all detailed anatomical description. Nevertheless we 

 must dwell at rather more length on the physiological province 

 of respiration, on the indispensable aid which it brings to the 

 work of nutrition in the complex animals. 



