422 MAMMALIA— PHYSIOLOGY 



bringing about rapid readjustments to external conditions, as well 

 as in keeping the natural relations between the systems and organs. 

 The organs of digestion include the alimentary canal and its accom- 

 panying secretory glands, the liver and pancreas, which have as 

 their function the preparation and absorption of foodo 



The organs of respiration comprise the lungs and skin. Their 

 function is the exchange of gases between the organism and the 

 atmosphere. The organs of circulation are the heart, arteries, veins, 

 and capillaries with fluid contents. Their function is distribution 

 and renovation of blood throughout the organism. The muscle fiber 

 is the essential element of this system. The main organs of excretion 

 are the kidneys, which are supplemented by the action of the lungs, 

 the skin, the liver and the blood. Their function is the separation 

 of urea from the blood. The active element is the epithelial cell. 

 The organs of reproduction are the ovaries and the testes. The essen- 

 tial elements are the ovum and the spermatozoan, both of which 

 are derived originally from epithelial cells. 



The muscular and nervous systems form an organ which is both 

 master and servant to the entire organism. The nerve centers 

 receive information through the sense organs and nerves regarding 

 the outer and inner needs of the entire organism and, acting through 

 the motor nerves^ bring about muscle activity producing those move- 

 ments that satisfy the demands of the situation. 



The principal nerve centers are the brain and spinal cord. The 

 principal sense organs are the skin, eye, ear, nose and tongue. 

 Many of our most important body functions are carried on, without 

 fret or worry, by the sympathetic system. 



Mammalian Physiology 



External Anatomy and Locomotion. — For the most part the 

 mammals are quadrupedal in locomotion. Even in man, the biped, 

 whose upright posture has been fashionable for ages, it has been 

 shown that resumption of the quadrupedal method of locomotion 

 may benefit cases of prolapsed viscera. 



The plantigrade animals like man and the bear are usually five- 

 toed. The digitigrade forms such as the cat are four-toed. In the 

 horse, a digitigrade or unguligrade animal, the original four toes 

 (with a rudimentary fifth) have degenerated to a single toe. The 

 horse and the antelope seem particularly adapted for speedy flight. 



