THE LARYNX. 217 



its attachment to the palatine bone, it will open but a little way, and that only 

 in one direction. It will permit a pellet of food to pass into the resophagus; but 

 it will close when any pressure is made upon it from behind. Two singular facts 

 necessarily follow from this : the horse breathes through the nostrils alone, 

 and these are capacious and easily expansible to a degree seen in no other 

 animal, and fully commensurate to the wants of the animal. 



It is also evident that, in the act of vomiting, the contents of the stomach 

 must be returned through the nostril, and not through the mouth. On this 

 account it is that the horse can with great difficulty be excited to vomit. 

 There is a structure at the entrance to the stomach which, except under very 

 peculiar circumstances, prevents its return to the throat, and consequently to 

 the mouth. 



The muscles of this singular curtain are very intelligibly and correctly 

 described by Mr. Percivail, in his " Anatomy of the Horse," to which the 

 reader is referred. The same remark is applicable to a very singular and 

 important bone, and its muscular apparatus, the os hyoides. 



THE LARYNX 



Is placed on the top of the windpipe (see 1, p. Ill), and is the inner guard of 

 the lungs if any injurious substance should penetrate so far ; it is the main 

 protection against the passage of food into the respiratory tubes, and it is at the 

 same time the instrument of voice. In this last character it loses much of its 

 importance in the quadruped, because in the dumb animal it is a beautiful piece 

 of mechanism. 



THE EPIGLOTTIS (see 2, p. HI) is a heart-shaped cartilage, placed at the 

 extremity of the opening into the windpipe, with its back opposed to the pha- 

 rynx, so that when a pellet of food passes from the pharynx in its way to the 

 oesophagus, it presses down the epiglottis, and by this means, as already 

 described, closes the aperture of the larynx, and prevents any portion of the 

 food from entering it. The food having passed over the epiglottis, from its own 

 elasticity and that of the membrane at its base, and more particularly the 

 power of the hyo-epiglotideus muscle, rises again and resumes its former 

 situation. 



THE THYROID CARTILAGE (see 1, p. Ill) occupies almost the whole of the 

 external part of the larynx, both anteriorly and laterally. It envelopes and 

 protects all the rest ; a point of considerable importance, considering the injury 

 to which the larynx is exposed, by our system of curbing and tight reining. It 

 also forms a point of attachment for the insertion of the greater part of the 

 delicate muscles by which the other cartilages are moved. 



The beautiful mechanism of the larynx is governed or worked by a some- 

 what complicated system of muscles, for a description of which the reader is 

 referred to the 5th vol. of The Veterinarian, p. 447. It is plentifully supplied 

 with nerves from the respiratory system, and there are also frequent anastomoses 

 with the motor nerves of the spinal cord. The sole process of respiration is 

 partly under the control of the will, and the muscles of the larynx concerned 

 in one stage of it are likewise so, but they also act independently of the will, 

 for during sleep and unconsciousness the machine continues to work. 



The origin of the artery which supplies these parts with blood is sometimes 

 derived from the main trunk of the carotid, but oftener it is a branch of the 

 thyroideal artery. 



The lining membrane is a continuation of that of the pharynx above and the 

 trachea below. It is covered with innumerable follicular glands, from whose 

 mouths there oozes a mucous fluid that moistens and lubricates its surface. H 



