xi URIXOGEXITAL ORGANS 537 



arising from the wall of the tympanic cavity and inserted 

 in to the manubrium of the malleus. 



The study of development indicates that the malleus 

 corresponds to the ossified articular part of the mandibular 

 cartilage of lower Vertebrates, and the incus to the quadrate 



Ex 



FIG. 143. Diagram of the mammalian body labyrinth, tympanic cavity, and external 



auditory passage. 



Cch. bony cochlea ; E. Eustachian tube ; Ex. external auditory passage ; L. bony 

 semicircular canals ; M. tympanic membrane; .V. auditory nerve; Oj stapes ; 

 O-i incus ; 03 malleus. (After Headley.) 



(pp. 44 and 438) : the articulation of the bony mandible 

 with the squamosal in Mammals has rendered these parts 

 unnecessary for their original purpose ; and they have 

 undergone a change of function (p. 449), forming an accessory 

 part of the auditory apparatus. 



Urinogenital Organs. The kidneys (Fig. 135, k) are 

 of a somewhat compressed, oval shape, with a notch or 

 hilns on the inner side. They are in close contact with 

 the dorsal wall of the abdominal cavity, the right being 

 somewhat in advance of the left. Towards the hilus, 



PRACT. ZOOL. 



L L 



