320 



THE LAPVNX, LUNGS, VOCAL SACS, ETC. 



III. THE VOCAL SACS. 



VH 



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 xTnder the angle of the mouth 

 and tympanic membrane. In well- 

 develojied 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 

 consists of connective-tissue, with 

 a large proportion of yellow elastic 

 fibre. Internally it is lined with 

 a flattened epithelium, and exter- 

 nally is covered with a layer of striated muscular fibre, deri-\Td 

 from the mylo-hyoid muscle (Fig. 209 J\f//, J^f'/). 



H II 



Dissection to s)iow the vocal sac of the 

 right side. 



//// Larger posterior corniia of the liyoid. 



////I Smaller posterior cornuaof the hyoid, 



My Mylo-hyoid muscle, 



il/j/1 Mylo-hyoid muscle continued on ti 



the vocal sac. 



Tlui Thyroid glands, 



VII Anterior cornua of the hyoid. 



Z Body of the hyoid . 



IV. THE THYMUS GLAND. 



a. General description. The thymus gland(Fig. 210 Th) 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 31. depressor tnandihiilae {JD.iii.). The 

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

 long, lying in the space between the M. (lejjressor luandihuJae and 

 the J/, sternocleidomas toideits [St) ; it extends slightly beyond the 

 posterior border of the former muscle. The space also includes 

 connective-tissue, fat, and numerous vessels. 



In liana teniporaria this gland is spherical, much smaller, and 

 placed further behind on the M. sternodeidomastoideits^ between the 

 M. latissmns dorst and the ]\f. delfoidens (Wiedersheim). 



