536 RESPIRATORY APPARATUS IN MAMMALIA 



the act of respiration, the larynx does not give rise to any very interesting 

 physiological considerations. It is, nevertheless, worthy of remark that this 

 organ, like the nostrils, dilates or contracts, according to the volume of the 

 column of air introduced into, or expelled from, the lungs, and that its paralysis, 

 during rapid movements, causes an embarrassment in the respiration which 

 betrays itself in " roaring." But a physiological study of the larynx acquires a 

 real interest when it is examined with regard to the articulation of sounds, or as 

 an organ of phonation. This study, however, does not come within our province ; 

 though what has been said concerning the vocal cords will give a summary, but 

 satisfactory, idea of the mechanism which presides over this function, and the 

 part the larynx plays. 



It may also be added, that nearly all the muscles of the larynx are concerned 

 in phonation, as by modifying the tension and the separation of the vocal cords, 

 they determine differences in the sounds. One only is concerned in respiration ; 

 this is the posterior crico-arytaenoideus, which is a dilator of the glottis.^ 



2. The Trachea (Figs. 317, 318). 

 Preparation. — Follow the same procedure as for the dissection of the oesophagus. 



The trachea is a flexible and elastic tube, formed by a series of incomplete 

 cartilaginous rings that succeed the larynx, and terminate above the base of the 

 heart by two divisions, which constitute the bronchi. 



Form. — This tube is cylindrical, and (slightly) flattened above and below. 

 Its inferior face and two borders are regularly rounded, and offer transverse 

 grooves, which correspond to the intervals between the constituent pieces of the 

 trachea. The superior face, nearly plane, shows the thin and widened extremities 

 of these cartilages. 



Course. — Leaving the posterior extremity of the larynx, the trachea descends 

 backwards to the entrance of the chest, in following the inferior border of the 

 neck below the longus colli muscle. It afterwards becomes inflected, superiorly^ 

 to pass between the two first ribs, enters the chest, passes through the anterior 

 mediastinum, proceeds directly backwards, and finally arrives above the left 

 auricle of the heart, to the right of the posterior aorta, where it bifurcates. 



Relations. — In its cervical portion, the trachea, surrounded by a loose and 

 abundant connective tissue, lies in a kind of muscular envelope which the 

 majority of the muscles of this region form around it, and which are : the 

 sterno-hyoid and sterno-thyroid, placed in front ; the sterno-maxillaries, situated 

 at first in front, and afterwards on the sides towards their termination ; the 

 subscapulo-hyoideii, above and in the middle of the lateral parts ; the scalenii, 

 altogether below and at the sides ; the longus colli, behind ; and outside all 

 these muscles, the superficial expansion of the cervical panniculus. This enve- 

 lope is thinnest in front of the middle portion of the neck ; and this is the part 

 where the operation of tracheotomy should be performed. 



The trachea is also in relation, in its cervical portion : 1. With the oeso- 

 phagus, which descends, as we know, at first in the middle of the posterior face, 

 then to the left side of the air-tube. 2. With the carotid arteries, which pass 

 along both sides of the tube, accompanied by their satellite nerves — the pneumo- 

 gastric, great sympathetic, and recurrent nerves. 



(' For a special and detailed description of the anatomy and physiology of the Horse's 

 larynx, consult my work on Roaring in Bursts {LitryngiKsimus paralyticus). London : 1889.) 



