176 OUTLINES OF EQUINE ANATOMY. 



ventricles of the larynx, which are suj)posed to give 

 resonance to the voice ; they j^ass down between the thyro- 

 arytenoidei anticus and posticus, and in the upper part of 

 each in the ass we see a small opening leading into a 

 secondary cavity. After passing over the cordse vocales 

 the mucous membrane lines the inner surface of the cricoid 

 cartilage, and is continued downwards into the trachea. 



Thyro-hyoideus arises from the external surface of the 

 wing of the thyroid, and runs to be attached to the heel 

 process of os hyoides ; its internal surface is in contact 

 with the thyro-hyoid ligament. 



Crico-thyroideus runs from the external surface of the 

 lateral part of the cricoid to the posterior part of the inferior 

 border of the wing of the thyroid. 



Crico-arytenoidens posticus arises from the superior 

 surface of the posterior hexagonal part of the cricoid, and 

 its fibres converge to become inserted into the central 

 ridge on the external surface of the arytenoid, in common 

 with 



Crico-arytenoideus lateralis, which arises from the 

 superior margin of the cricoid at the lateral part. Ary- 

 tenoidens runs from the external surface of one arytenoid 

 to that of the other, consisting really of two muscles 

 blending along the central line. 



Thyro-arytenoidei posticus and anticus (generally con- 

 sidered as one muscle) are separated by the inflection of 

 mucous membrane, which forms the ventricle of the larynx. 

 They arise from the internal surface of the thyroid, and 

 run to the anterior part of the external surface of the 

 arytenoid. 



The trachea is a large tube through which air passes in its 

 l^assage from the larynx into the lungs and vice versa. It is 

 composed of a series of cartilaginous rings, fifty to sixty in 

 number, which are embedded in and connected together by a 

 quantity of yellow elastic tissue. These rings are thickest in- 

 feriorly,and superiorly are deficient, being terminated by two 

 thin expanded extremities which overlap each other, and are 

 connected together by loose areolar tissue. Anteriorly the 

 trachea commences by being connected to the inferior mar- 

 gin of the cricoid cartilage by the crico-trachealis ligament y 

 which allows considerable motion between the two. Pos- 

 teriorly after crossing the base of the heart in the thorax^ 

 where it is situated between the large arteries at their com- 



