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Anatomy, Physiology, and Pathology of the 



The ingress and egress of the air in respiration also differs. 

 In the horse each is carried on through the nasal passages, ex- 

 cept in coughing, when a portion of the air is expelled by the 

 mouth. But in the ox and sheep the air enters and escapes both 

 by the mouth and nostrils. This variation in part depends on 

 the situation of the larynx with reference to the velum palati, and 

 also on the length and position of the velum ; peculiarities which 

 can only be alluded to. 



The lower part of the larynx is continuous with the windpipe, 

 which is likewise composed of a series of cartilages arranged 

 in a circular order. The windpipe, in common with the other 

 portions of the respiratory passages, is lined with a mucous 

 membrane, the secretion of which defends these parts from the 

 irritation of the atmospheric air. This membrane not unfre- 

 quently suffers from a change of temperature, &c, and is the 

 seat of those diseases recognised as catarrh, laryngitis, bron- 

 chitis, &c. The number of the rings of the windpipe will 

 of course be governed by the length of neck : in the ox we 

 usually find from 55 to 60. These rings are greater in sub- 

 stance at their front, being here more exposed to external in- 

 juries, than at their hinder part. They are united to each other 

 by elastic tissue, which allows the windpipe without inconvenience 

 to accommodate itself to the various movements of the neck. 

 On the inner and back part of the rings, lying between them and 

 the mucous membrane, is a thin layer of muscular fibres, the 

 use of which is one of the vexed questions of physiology. The late 

 Mr. Youatt denied the existence of this muscle in the ox ;* 

 I have satisfied myself, however, that it not only exists in this 

 animal, but in every other which hitherto I have examined. 

 It is a singular fact, and one which I am desirous of naming 

 in this place, that in the dog the muscle is situated on the 

 outer, and not on the inner, part of the windpipe. Mr. Percival 

 is of opinion that the muscle resists the tendency of the elastic 

 cartilaginous rings to form an elliptical-shaped canal ; and, by 

 converting the ellipsis into a circle, may thus tend to expand 

 and not to contract the calibre of the tube.f Whether it be 

 so in moderate action of the muscle or not, it is clear that when 

 its fibres are contracting with energy, or are unduly stimulated, 

 it must diminish the area of the canal. I would ask if it be not 

 specially employed in the lower animals, in whom the vocal appa- 

 ratus is exceedingly simple, compared with man, for producing 

 the voice, by regulating the volume of the exhaled air, entering 

 the larynx from the lungs ; and also whether some of their into- 

 nations do not depend on the amount of its action ? 



* See 4 Cattle, 1 p. 374, Society for the Diffusion of Useful Knowledge, 

 f Anatomy of the Horse, p. 225. 



