HEAD AND NECK OF THE HORSE 97 



the larynx is firmly adhereut to the dorsal surface of the epiglottis, the 

 medial surfaces of the arytenoid cartilages and over the vocal folds, 

 where it is thin. Elsewhere in the vestibule and about the aditus it is 

 thicker and loosely adherent to the subjacent structures. 



Dissection. — Remove the remains of the pharyngeal, sterno-thyroid, 

 sterno-hyoid and thyro-hyoid muscles. Take care to preserve the laryngeal 

 nerves and vessels. 



Now examine the structures on the ventral and lateral aspects of the 

 larynx. 



The connection of the hyoid boue and the thyroid cartilage is 

 established by the articulation of the extremities of the thyroid 

 processes of the bone with the cranial cornua of the thyroid, and by the 

 presence of the hyo-thyroid membrane (membrana hyothyreoidea), 

 which fills the gap between the hyoid bone and the thyroid cartilage, 

 and is attached to the body and thyroid processes of the hyoid and to 

 the anterior border of the thyroid laminae. 



The joint between the bone and the thyroid cornu (articulatio 

 hyothyreoidea) is a diarthrosis and is provided with a thick 

 joint capsule. The movement in the joint is of the nature of a rotation 

 about a transverse axis passing through the joints on the two sides of 

 the larynx. 



M. CRICOTHYREOIDEUS. — The crico-thyroid muscle lies in the groove 

 on the cricoid arch, from the lateral surface of which cartilage it takes 

 its origin. Its insertion is into the caudal border and immediately 

 adjacent part of the lateral surface of the thyroid lamina. By rotation 

 of the cricoid cartilage about a transverse axis passing through the 

 right and left cricothyroid joints, the muscle moves the anterior border of 

 the cricoid lamina — and the bases of the arytenoid cartilages situated 

 thereon — upwards and backwards, and thus tenses the vocal folds. 



The thin and membranous crico-thyroid ligament (ligamentum 

 cricothyreoideum) is visible between the two crico-thyroid muscles, and 

 fills the triangular interval between the thyroid and cricoid cartilages. 

 The joint between the caudal thyroid cornu and the cricoid cartilage 

 should be looked at now, but its complete examination may be 

 deferred until the dissection of the larynx nears completion. The joint 

 is a diarthrosis provided with the usual joint capsule attached round the 

 articular margins of the two cartilages. 



Dissection. — IS ow turn to the dorsal aspect of the larynx. Remove 

 the mucous membrane from the outer surface of the cricoid, arytenoid and 

 corniculate cartilages, taking care not to destroy the cranial laryngeal and 

 recurrent nerves. The origin of longitudinal muscular fibres of the 

 oesophagus from the interval between the cricoid and arytenoid cartilages 

 7 



