584 



ZOOLOGY 



SECT. 



passage the Eustachian tube (E.). On its inner wall are the 

 fenestrcB ovalis and rotunda, and across its cavity, from the tympanic 

 membrane to the fenestra ovalis, runs the irregular chain of auditory 

 ossicles the malleus (0 3 ), the incus (0. 2 ) and the stapes (OJ. 

 These vary somewhat in form in different" Mammals. The stapes 

 is usually perforated by a considerable foramen, as in the Rabbit, 

 but, in the Monotremes, certain Marsupials, and Manis among the 

 Edentata, approximates more towards the rod-like shape which 

 the columella presents in Amphibians, Reptiles, and Birds. The 

 membranous labyrinth (L.) of the internal ear of a Mammal is 

 characterised by the special development of the cochlea (Cell.}, 

 which (except in the Monotremes) is coiled into a spiral like the 

 shell of a Snail. 



Urinogenital Organs. The kidneys of Mammals are compact 



organs of oval 

 shape. On the 

 inner side is a 

 notch or hilus, 

 by which vessels 

 and ducts enter 

 or leave the in- 

 terior of the 

 kidney. The 

 substance of the 

 kidney consists 

 of two distinctly 

 marked portions 

 a central por- 

 tion or medulla, 

 and an outer part 

 or cortex ; the 

 latter is the 

 secreting part ; 

 the former con- 

 sists of a mass of 

 straight tubules 



by which the secretion is carried to the ureter. The ureter 

 dilates as it enters the kidneys to form a chamber the pelvis 

 into which the straight tubules of the medulla of the kidney 

 open. The openings of the tubules are on the summits of papillae, 

 which are the apices of a series of pyramidal masses into which, 

 in most cases, the substance of the kidney is incompletely divided. 

 In many Mammals, however, there is no such division of the kidney 

 substance, and all the ducts open on the surface of a single papilla. 

 In others again (Ox, Bears, Seals, Cetacea) the division is carried 

 so far that the kidney is divided externally into a number of 

 distinctly separated lobules. 



Fio. 1222. Parts of the Human ear>( diagrammatic). Cch. cochlea ; 

 E. Eustachian tube ; Ex. outer opening of ear ; L. labyrinth ; 

 M. tympanic membrane ; N. entrance of auditory nerve ; Oi, Og, 

 O 3 , the three auditory ossicles, stapes, incus, malleus. (After 

 Headley.) 



