xm PHYLUM CHORDATA 549 



wards to the tympanic membrane (M.), which separates it from the 

 cavity of the middle ear or tympanic cavity. The wall of the ex- 

 ternal auditory passage is sometimes entirely membranous or 

 cartilaginous, sometimes in part supported by a tubular part of 

 the tympanic bone; m Echidna it is strengthened by. a series of 

 incomplete rings of cartilage. The tympanic cavity, enclosed by 

 the periotic and tympanic bones, communicates with the upper or 

 respiratory division of the pharynx by a longer or shorter tubular 

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

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

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



Ex 



FIG. 1141. 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 ; 

 0\. 0%. 6*3. the three auditory ossicles, stapes, incus, malleus. (After Headley.) 



ossicles the malleus (<9 3 .), the incus (0 2 .), and the stapes (0 r ). 

 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 and Marsupials, approximates more to- 

 wards the rod-like shape which the columella presents in Am- 

 phibia, Reptiles, and Birds. The membranous labyrinth (Z.) of 

 the internal ear of a Mammal is characterised by the special 

 development of the cochlea (Cch.}, which (except in the Monotremes) 

 is coiled into a spiral like the shell of a Helix. 



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 interior of the kidney. 

 The substance of the kidney consists of two distinctly marked 

 portions a central portion or medulla, and an outer part or cortex ; 

 the former is the secreting part, the latter consists of a mass of 



