MAMMALIA. 41 



upper extremity of the trachea ; a fleshy curtain, called the 

 <elum palati^ establishes a direct communication between their 

 larynx and nasal canal. 



Their residence on the surface of the earth rendering them 

 less exposed to the alternations of cold and heat, their tegu- 

 ment, the hair, is but moderately thick, and in such as inhabit 

 warm climates, even that is rare. 



The Cetacea, which live exclusively in water, are the only 

 ones that are altogether deprived of it. 



The abdominal cavity is lined with a membrane called the 

 peritoneum, and the intestinal canal is suspended to a fold of 

 it called the mesentery, which contains numerous conglobate 

 glands in which the lacteals ramify: another production of 

 the peritoneum, styled the epiploon, hangs in front of and un- 

 der the intestines. 



The urine which is retained for a time in the bladder finds 

 an exit in both sexes, with very few excepJ:ions, by orifices 

 in the organs of generation. 



In all the Mammalia, gener?ftion is essentially viviparous; 

 that is, the foetus, directly after conception, descends into the 

 uterus enveloped in its membranes, the exterior of which is 

 called chorion and the interior amnios; it fixes itself to the 

 parietes of this cavity by one plexus, or more of vessels called 

 the placenta, which establishes a communication between it 

 and the mother, by which it receive;^ its nourishment, and most 

 probably its oxygenation, notwithstanding which, the foetus 

 of the Mammalia, at an early period, has a vesicle analogous 

 to that which contains the yolk in the Ovipara, receiving in 

 like manner vessels from the mesentery. It has also another 

 external bladder named the allantoid, which communicates 

 with the urinary one by a canal called the urachus. 



Conception always requires an effectual coitus, in which the 

 semen masculinum is thrown into the uterus of the female. 



The young are nourished for some time after birth by a 

 fluid (milk) peculiar to animals of this class, which is pro- 

 duced by the mammae at the time of parturition, and continues 

 to be so as long as is necessary. It is from the mammx that 

 Vol. I. F 



