320 



THE LARYNX, LUNGS, VOCAL SACS, ETC. 



III. THE VOCAL SACS. 



Fig. 209. 



a. General description. The vocal sacs are a pair of sacs 



which open in the floor of the 

 mouth (Fig. 179 S) ; they are found 

 only in the males. When the 

 animal croaks these sacs are di- 

 lated and act as resonators ; when 

 so dilated the sacs force up the 

 skin under the angle of the mouth 

 and tympanic membrane. In well- 

 developed specimens they are about 

 as large as an average sized cherry. 

 The skin covering the sacs is ex- 

 tremely elastic, but is not directly 

 attached to the sacs. 



b. Minute structure. The sac 



Thy 



Dissection to show the vocal sac of the 

 right side. 



HH Larger posterior cornua of the hyoid. 



7f//i Smaller posterior cornua of the hyoid. 



My Mylo-hyoid muscle. j i ,-t 



Myl Mylo-hyoid muscle continued on to COUSlStS Ot COnnectlVC-tlSSUC, With 



the yocai sac. a i ar . e proportion of yellow elastic 



Thy Thyroid glands. , * 



VH Anterior cornua of the hyoid. fibre. Internally it is lined with 



Z Body of the hyoid . a ,, , . , ,. , 



a flattened epithelium, and exter- 

 nally is covered with a layer of striated muscular fibre, derived 

 from the mylo-hyoid muscle (Fig. 209 My, My^}. 



IV. THE THYMUS GLAND. 



a. General description. The thy mus gland (Fig. 210 T/t .) is placed 

 on each side behind the angle of the jaw; it is best exposed by re- 

 moving* the skin behind the tympanic membrane and the angle of 

 the jaw, and then reflecting the M. depressor mandilulae (D.M.). The 

 gland is then found as an elongated, oval body, not quite 3 mm. 

 long, lying in the space between the M, depressor mandilulae and 

 the M. sternocleidomas toideus (St)-; it extends slightly beyond the 

 posterior border of the former muscle. The space also includes 

 connective-tissue, fat, and numerous vessels. 



In Rana temporaria this gland is spherical, much smaller, and 

 placed further behind on the M. sternocleidomastoidens, between the 

 M. latissinnis dorsi and the M, deltoid ens (Wiedersheim). 



