462 



MAMMALIA— PHYSIOLOGY 



urinary bladder. From thence it is transported by means of the 

 urethra to the exterior. The kidneys expel water, urea and a little 

 CO2. The average amount of urine secreted in man per day is 

 fifty ounces. The lungs exhale CO2 and water with a trace of urea, 

 while the skin excretes as the lungs do and also functions in place of 

 the kidneys when they are diseased (see Perspiration, page 428). 

 The liver, by means of its hepatic portal system (see page 454), 

 separates out the impurities from the blood and passes them into 

 the duodenum. Intestinal excretion plays an important role in the 

 economy of the animal. 



Fig. 248. Graafian follicle and ovum of the cat, within the ovary. (Photograph 

 through the courtesy of Dr. B. F. Kingsbury.) 



Reproductive System. — While in the lower vertebrates there is 

 an extremely close relationship between the urinary and the repro- 

 ductive system, evidenced in the frog, for instance, by the passage 

 of sperms through the kidney and out through the common Leydig's 

 duct, we find that in the mammal, at birth, such connection largely 

 disappears. 



Female. — In the cat there are two small oval ovaries^ which 

 produce Graafian follicles and send the ova therein contained down 

 the elongated Fallopian tubes, the lower portions of which, as horns 



