X 



Ivi Introduction. 



by an epiglottis, and with the exception of Bradypus tridacti/Uts, 

 the trachea always takes a direct antero-posterior course from the 

 larynx to its bifurcation. The lungs are always freely suspended 

 in pleural cavities, and they are never prolonged into abdominal or 

 other air-sacs. A perfect diaphragm is always pi'esent. Portions 

 of this and of other muscles are always interposed between the 

 kidneys and the lower dorsal and upper lumbar vertebrae in the 

 region of which they lie ; and their external surfaces are, conse- 

 quently, not conformed, as in other Vertebrata, so as to fit into the 

 sinuosities of the osseous structures in their neighbourhood. The 

 kidneys are provided with a fibrous envelope, surrounded by a 

 jpannicnlus adiposus, the venous system of which is in anastomotic 

 connection with that of the gland ; but this connection never 

 attains to the functional importance of a '■ renal-portal ' sj^stem. 

 Similar anastomoses, possessed similarly of merely morphological 

 importance, exist between the renal arteries, which bring to the 

 gland the blood upon which its secretion as well as its nutrition is 

 entirely dependent, and certain branches of the lumbar arteries. 

 The substance of the gland is always differentiated into an external 

 cortical secretory, and an internal medullary excretory stratum. 

 A urogenital canal, which is only occasionally found or rudi- 

 mentarily represented in other Vertebrata, is always found in 

 Mammals, except in the females of some llodentia or Insectivora, 

 where the clitoris forms a closed tube for the urethra. 



The cerebral hemispheres, as distinct from the corpora striata 

 and optic thalami which they overlie, attain a greater development 

 than in any other class of Vertebrata. Their external surfaces are 

 in many small, and in most large representatives of the Class, 

 convoluted, so as to allow of the ready access of blood to the sub- 

 stance of the hemispheres, at the same time that the amount of the 

 grey matter is greatly increased. The cerebral hemispheres are 

 always connected by a more or less extensive ' corpus callosum,' 

 and the mesencephalon is always represented by more or less 

 sharply separated ' corpora quadrigemina.'' In a few of the true 

 Cetacea the olfactory bulbs are absent, and in certain burrowing 

 Rodentia and Insectivora, the eyes may be absent or rudimentary, 

 but in all other Mammals the organs of special sense are all 

 present. Special organs of tactile sensibility are very ordinarily 

 developed upon the snout, as in the Carnivora and SoUdungula. 



