582 ZOOLOGY 



SECT. XIII 



inwards to the tympanic mcmlircine (M,\ which separates it from 

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

 the external auditory passage is sometimes entirely membranous or 

 cartilaginous, sometimes in part supported by a tubular portion of 

 the tympanic bone ; in 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 

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

 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 Mania 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 (Cch.\ 

 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 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 latter is the secreting part ; the former consists 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. 



The ureters in all the Theria open into a large median sac the 

 urinary Madder situated in the posterior or pelvic part of the 

 cavity of the abdomen. From this a median passage, the urino- 

 genital passage or urethra into wliich in the male the vasa 

 deferentia open leads to the exterior. Only in the Monotremes 

 do the two ureters and the bladder all have separate openings 

 into the urinogenital division of the cloaca. 



The testes are oval bodies, which only exceptionally retain their 

 original position in the abdominal cavity, descending in the 



