. 



MECHANICAL PHENOMENA OF RESPIRATION. 301 



orifice of the nasal chambers. Consequently, when the facial 

 nerve of a horse, which innervates the muscles of the nos- 

 tril, is cut, the nostril becomes inert and collapsed, and, as in- 

 spiration or expiration takes place, acts like a valve ; so that, 

 even if the animal opens the mouth wide, he is asphyxiated, 

 in spite of his efforts to breathe. This effect is peculiar to 

 the horse, not appearing in the dog or any other animal 

 which breathes through the mouth (Cl. Bernard). Finally, 

 in the human foetus, as well as in the foetus of the dog, it is 

 observed that the larynx extends a little higher than in the 

 adult, exhibiting, up to a certain point, the same disposition 

 as that which we have just described in the lower mam- 

 mals. 



The larynx, the trachea and its divisions, and the bronchi, 

 form a ramified tube, which, like all the constituent parts of 

 the respiratory system, is distinguished by its elastic elements. 

 These are, first, its cartilaginous rings^ which are incom- 

 plete behind ; the space left open, however, at the back ot 

 these rings, is filled by longitudinal bands of elastic tissue, 

 interlaced and anastomosed under the mucous coat. Deeper 

 down, the loose ends of each ring are joined together by 

 smooth muscular fibres; which continue as far as the last 

 bronchial ramifications, so that the last cartilaginous nuclei, 

 the remains of the tracheal rings, have already disappeared 

 while the muscular fibres are still found, in greater numbers 

 even, and more uniformly arranged, all around the smaller air 

 tubes (see p. 281) ; these fibres (muscles of Reisseisen) do 

 not contract at will. We may repeat, in regard to them, 

 what we have said of the muscular fibres of the alveolar wall 

 about which some doubt exists; for there may be other 

 muscular elements in the lungs than in the small bronchi and 

 the small vessels. It is difficult, if not impossible, to prove 

 that these fibres contract in order to take a share in physio- 

 logical actions. Their participation * in pathological phenom- 

 ena is also doubtful, as, for instance, they do not contract 

 with sufficient force to assist in coughing ; the possibility of 

 their intervention in asthma and bronchial spasms we have 

 already noticed. At all events, what we must recognize in 

 this element, as in the preceding, is an eminently elastic 

 tissue, whose chief use pertains to this property. Thus the 

 tracheal and bronchial cartilages resist the great changes of 

 shape, and restore the tube to its original form, when this has 



1 See note 2, p. 290. 



