68 U.S. NATIONAL MUSEUM BULLETIN 2 73 



ficial. As the muscle approaches the upper border of the thyroid 

 plate the dorsal fibers of the now much broadened thyrohyoideus 

 override the anterior fourth of the costothyroideus. It ends on the 

 outer lip of the upper thyroid border along its rear third and by its 

 deeper fibers on the adjacent part of the external surface of the plate. 

 Sandifort (1834) and Sirena (1871) described an intermediate tendon 

 in the muscle. The latter author gives only the first rib and costal 

 cartilage as points of origin. Hill (1962) believes that this muscle 

 replaces the omohyoideus. The interpretation of Starck and Schneider 

 (1960) was given in the section dealing with m. sternothyroideus. 



Nerve supply: It is commonly supplied by two branches of the 

 ansa hypoglossi, an upper one given off the ansa and a lower from the 

 long descending ramus to the sternohyoideus (fig. 23). 



Function: In combination with the sternothyroideus it retracts 

 the thyroid cartilage against the cricoid and trachea. It is conceivable 

 that the contraction of the two costothyroideus muscles wUl spread 

 the upper margins of the plate out and downward. The thyrohyoid 

 space and that circumscribed between the thyroid cartilage and the 

 epiglottis will thus be enlarged and a vacuum created within the sac- 

 cules and the hyolaryngeal canal, which will be immediately invaded 

 by an inrushing column of air. In this manner the antrum of the bulla 

 is also filled with air and the vibrations of the soundproducing struc- 

 tures can reach these chambers to be amplified. 



M. thyrohyoideus: This muscle, the cranial continuation of m. 

 sternothyroideus (fig. 3) , arises from the posterior third and outer lip 

 of the lower thyroid margin. Its superficial fibers are continuous 

 with those of the sternothyroideus. The flattened beUy covers the 

 thyroid attachment of the inferior pharyngeal constrictor and lies 

 initially under the costothyroideus. The thyrohyoideus expands when 

 crossing the upper thyroid margin and its dorsal fascicles now cover 

 the terminal part of the costothyroideus. Insertion is by fleshy fibers 

 on (1) the proximal hyoid part of the cornu branchiale I and (2) the 

 tentorium bullae between the hypohyal and the cornuhyoid joint. 

 The muscle is broader at its insertion than at the origin, the propor- 

 tion being about 2:1. The findings of other authors (Lampert, 1926; 

 Sandifort, 1834; Starck and Schneider, 1960) do not differ from mine. 



Nerve supply: It is usually supplied by a branch of the hypoglos- 

 sus nerve, but it may also be innervated directly from the descendens 

 liypoglossi or from a small ansa between cranial nerve XII and the 

 upper branch to the sternohyoideus (fig. 23). 



Function: Contraction of this muscle draws the cornu branchiale 

 I toward the upper border of the thyroid cartilage. At the same time 

 it presses against the flexible wings of the plate. A squeezing action 

 can then be exerted on the saccules contained in front of the epiglottis 



