THE SYRINX OF GALLUS DOMESTICUS 175 



the trachea at the level of the tenth ring (fig. 2). Here, as be- 

 yond, both muscles are firmly bound to the trachea by a strong 

 common fascial sheath. But despite their fascial attachment, 

 none of their fibers is inserted at this level. Springing from the 

 medial surface of both muscles are several small bundles of 

 fibers which, directed obliquely cephalad, are inserted on the 

 tracheal rings, from the thirteenth to the twenty-first. Ceph- 

 alad to these muscular bundles both muscles, now much dimin- 

 ished in size, proceed along the sides of the trachea to their final 

 insertion on the ventral cartilages of the upper larynx. The exact 

 extent and manner of insertion of the sterno-trachealis muscles 

 offers a field for further investigation. The action of these 

 muscles will be discussed in the section on function. 



Two pairs of short bundles of muscle fibers are found on the 

 caudal end of the trachea. One pair (fig. 2) lies on the ventro- 

 lateral aspect of the trachea, the other on its dorso-la*teral aspect. 

 Caudally these bundles end at the levels of the fifth to eighth 

 rings. Cephalically the majority of their fibers enter, and 

 apparently become a part of, the ventral and the dorsal margins 

 of the sterno-tracheales respectively. The fibers which do not 

 enter the sterno-trachealis muscles are inserted on the respec- 

 tive ventral and dorsal walls of the trachea. Some of the fibers 

 of the ventro-lateral bundles spread out cephalad to the level 

 of the twelfth ring, to cover the ventral surface of the trachea, 

 and certain fibers from the dorso-lateral bundles also spread out 

 to cover its dorsal surface. 



d. Sexual differences. Sexual differences are very marked in 

 song birds, especially as regards the size of the labia and the 

 syi'ingeal muscles. In male aquatic birds the tympanum is a 

 large bony swelling projecting from the left side of the trachea, 

 while in females the tympanum, though present, does not pro- 

 ject beyond the tracheal walls. The main sexual difference to 

 be noted in the s>Tinx of chickens is one of size (cf. figs. 5 and 6 

 for which males and females of the same varieties were chosen). 

 Male birds are usually larger than females, and so some dif- 

 ference would naturally be expected in the size of their voice 

 organs. But this difference in size of individuals is not sufficient 



