664 t)»- HARRIS H. WILDER, 



9. Tlioraclco-byoideiis (Stern o -by oideus and others), 



(figs. 2, 4, sh). 



9 a. Adductores Arcnum (figs. 2 aa). 

 9 b. Omo-hyoideus. 



Preparation. These muscles belong to one system, the last 

 two not being completely differentiated from the first, and must be 

 treated together. The large, segmented mass reaching from the hyoid 

 apparatus to the pelvic girdle may be divided by the very small 

 sternum into an anterior and a posterior portion. The posterior por- 

 tion persists in higher Vertebrates as the Rectus abdominis, while from 

 the anterior mass are differentiated steruo-hyoideus, sterno-thyroideus, 

 thyro-hyoideus, and omo-hyoideus. Of these, the Rectus abdominis and 

 often sterno - hyoideus show , even in man , traces of their primitive 

 segmentation. In Siren, the omo-hyoideus is represented by a lateral 

 slip to the inner (under) side of the shoulder girdle, while more an- 

 teriorly, a pair of narrow bands, separating from the main muscular 

 mass, are inserted into the gill arches, and are known as the adduc- 

 tores arcuum. 



These, of course, do not persist in higher animals. 



Only one half of this system should now be prepared, leaving the 

 other half in position. Separate, without cutting, the lateral border 

 of the muscle from its surroundings. Upon the border, running an- 

 teriorly, will be seen the hypoglossal nerve which supplies this muscle. 

 At the same time notice the Ramus recurrens vagi, in its distribution 

 to constrictor arcuum branchiarum, a band-like muscle half surround- 

 ing the gills. Several other branches of this and other vagus stems 

 will be met with in this region. For farther preparation, free from 

 fascia the two adductores arcuum of one side, divide them in the 

 middle and turn them back; then cut away the fibres of thoracico- 

 hyoideus of the same side from their attachment to the hyoid appa- 

 ratus, leaving a few fibres about the branching end of basibranchiale 2. 

 When free, lift up the entire mass from the subjacent parts and 

 remove the whole by cutting through first the omo- hyoid slip, and 

 then the entire mass near the sternum. 



Anterior attachment, 



a) Thoracico-hyoideus. This muscle is attached 1) to the 

 lateral and internal surfaces, and the forked processes of basi- 

 branchiale 2. 2) To the medial half of the posterior surface 



