DR. J. MUEIE ON THE ANATOMY OF THE SEA-LION. 549 



from the great size and thickness of the thyro-arytenoidei muscles, are partially excluded 

 from the formation of the true vocal cords. 



The capsular ligaments encircling the synovial articulation between the posterior 

 (inferior) cornua of the thyroid and the postlateral facet of the cricoid cartilage are 

 short, and limit considerably the motion of the joint. The crico-arytenoid ligaments, 

 on the contrary, are wide, loose, and permit great freedom of motion of the arytenoid 

 cartilages, especially in an antero-posterior direction. The more elastic and resilient 

 cartilages of Santorini and Wrisberg do not possess any definite articulation or liga- 

 ments, but pass indefinitely the one into the other by cartilaginous union. There is, 

 moreover, a small but strong ligamentous union uniting the inner points of the aryte- 

 noid cartilages (interarytenoid ligament), and a less distinct, by reason of the inter- 

 mixture of muscular and fatty tissue, posterior crico-arytenoid ligament or connect^ 

 ing membrane. 



The superior thyro-arytenoid ligaments, or false vocal cords, are so much interwoven 

 with the submucous tissues, fat, and muscular fibres of the thyro-arytenoideus secundus 

 as to prevent their special dissection. The true vocal cords or inferior thyro-arytenoid 

 ligaments, however, are much better expressed. They pass in the usual manner from 

 the elevation behind the junction of the thyroid alse backwards to the inward or 

 anterior projection of the arytenoid cartilages and adjoining portions of the comicula 

 laryngis. 



The upper end of the trachea is firmly lashed all round the interior of the cricoid 

 cartilage by a very strong membrane, which, however, from its elastic nature, allows of 

 a certain amount of up and down movement. 



d. Muscles of the Os hyoides and Larynx. — The most superficial layer, in this case 

 long massive muscles connected with the hyoidean region — to wit, the omo-hyoid, sterno- 

 hyoid, and sterno-thyroid, forming a great part of the thickness of the neck and being 

 involved with the structures at their origins, have consequently been described in 

 Part II. 



Thyro-hyoid. — The direction of the fibres of this muscle are at an obtuse angle inwards 

 from those of the sterno-thyroid ; therefore there is a clear line of demarcation between 

 them. The thyro-hyoid is of considerable thickness, 2 inches long by 1 broad, and tra- 

 pezoidal in shape. It rests upon the fibres of the inferior constrictor muscle, outer 

 ala of the thyroid cartilage and the thyro-hyoid membrane ; and it is itself covered by 

 the omo-hyoid muscle. Its origin is the curved line and prominence of the thyroid 

 cartilage, and its insertion the inferior (posterior) border of the osseous thyrohyal and 

 the thyro-hyoid membrane. On the right side a slip of the inferior constrictor was 

 observed to pass across the thyro-hyoid just behind its middle. 



In Otaria juhata the Crico-thyroid is represented by a large and broad plane of 

 muscle, as a whole, quadrilateral in shape, though rather irregular in outline. It comes 

 from nearly the whole outer moiety of the cricoid cartilage, and covers the crico-thyroid 



VOL. VIII.— PART IX. June, 1874. 4g 



