A Contriliutioii to the Anatomy of Siren lacertînfl. QQ^ 



The muscles belonging to this group are six in number viz: two 

 pharyngeal constrictors; one, a part of the constrictor system diffe- 

 rentiated into a laryngeal muscle; and three genuine muscles of the 

 larynx. The largest and most superficial of these runs from the back 

 directly inwards, spreading itself out over the trachea. This is the 

 dorso-trachealis. A narrow band on the anterior margin of this 

 constitutes the dorso-laryngeus. The hyo-trachealis has 

 a course and position similar to that of dorso-trachealis, but is smaller 

 and forms a second and deeper layer, A pair of small muscles lying 

 at the base of the larynx and extending downwards upon the trachea, 

 form the depressor laryngis, A smaller pair superior to these, 

 and lying upon the larynx are the two constrictores laryngis, 

 while on the other (dorsal) side of the larynx are a similar pair, the 

 dilatatoreslaryngis. 



1) Dorso-trachealis (fig. 2, 6, 7 d. tr.). 



Origin and insertion. In Siren this muscle is very large, 

 completely covering the subjacent cucullaris. It arises from the fascia 

 dorsalis and passes around the ventral surface of the body, its fibres 

 diverging on the way. Of these fibres, the most superficial become 

 gradually more and more divergent, and, attaining an anterior position, 

 become the dorso-laryngeus, soon to be described. The remaining 

 fibres, divergent and fan-shaped, are inserted into the ventral surface 

 of the trachea, extending from the region of the heart to the larynx. 



Innervation. The muscle is supplied by fibres from themain 

 stem of Ramus recurrens vagi. 



2) Dorso-laryngeus (fig. 7 d.ly.). 



This muscle is best separated near the middle and the two portions 

 reflexed. The origin, common with that of the preceding muscle, has 

 been described. The fibres, however, soon take a different direction 

 from those of the muscle from which it originated, and attaining finally 

 a longitudinal direction, are inserted into the lateral process of the 

 arytenoid cartilage of the larynx. The innervation, like that of the 

 preceding muscle, is from the main stem of Ramus recurrens vagi. 



3) Hyo-trachealis (fig. lhj.tr.). 



The two preceding muscles may be lifted up and reflexed, thus 

 exposing the entire hyo-trachealis. 



The muscle arises from the middle third of the posterior border 

 of epibranchiale 4, and is inserted by divergent fibres, as in the case 



