66 U.S. NATIONAL MUSEUM BULLETIN 2 73 



development of this muscle. The last author and Sandifort (1834) 

 remarked that its transverse dimension was reduced from above 

 downward. 



Nerve supply: The sternohyoid eus receives two branches, one 

 giVen off either from the descendens hypoglossi, or the ansa, goes 

 to the upper part of the muscle, the other reaches the distal part 

 and is either a branch or the continuation of the long ramus of the 

 ansa hypoglossi to the sternothyroideus (fig. 23). 



Function: Its contraction should displace the hyoid away from 

 the mandible, and together with the stylohyoideus it must help to 

 close the hyolaryngeal canal by pressing the bulla against the ep- 

 glottis. By acting on the thyroid plates the sternohyoideus probably 

 helps to empty the saccules. 



M. sternothyroideus: It lies under cover of the sternohyoideus and 

 against the isthmus of the thyroid gland, the cricoid arch and m. 

 cricothyroideus. The deep lamina of the infrahyoid fascial layer 

 provides a cover for the sternothyroideus. Origin is by fleshy bundles 

 attached inside the thorax to (1) the medial end of the third costal 

 cartilage, sternocostal articulation and adjacent sternal pieces and 

 (2) the same regions at the level of the second cartilage below the 

 origin of m. sternohyoideus. The fleshy band ascends into the neck 

 immediately in front of the internal thoracic artery and other superior 

 mediastinal structures. It is inserted on the distal border of the thyroid 

 cartilage near the root of the lower horn (fig. 3). Sirena (1871) de- 

 scribes the origin only from the second costal cartilage and adjacent 

 sternal surface. Lampert's account (1926) of this muscle in the howler 

 does not include the origin because the specimens he studied had all 

 been beheaded. Sandifort (1834) points out how the ending of the 

 sternothyroideus on the thyroid cartilage does not cause any inferior 

 tubercle or linea obliqua. Starck and Schneider (1960) regard this 

 muscle as formed by two portions, one corresponding to the structure 

 I have just described, the other to the costothyroideus. 



Nerve supply: Distal twigs from the long branch of the ansa 

 hypoglossi (fig. 23). 



Function : Depressor of the thyroid cartilage. 



M. costothyroideus: This is a ribbon-like muscle found on the 

 lateral aspect of the larynx (fig. 3) and within the same fascial cover 

 as the sternohyoideus. It is broader at its insertion and begins by 

 muscular fibers from the deep surface of the lateral half of the first 

 and second costal cartilages. Near the origin the ventral border of 

 the costothyroideus is very near the lateral border of m. sternohy- 

 oideus and the two muscles appear to form a single sheet for a short 

 distance. They separate by following different directions and- leave 

 a space in between through which the thyrohyoideus becomes super- 



