96 THE NUTRITIVE FDNCTIOKS. 



dent of that of the medium in which they live, and are 

 called warm-blooded animals. But in reptiles and in 

 fishes the venous and arterial blood become mixed together, 

 the former being only partially returned to the lungs; 

 hence respiration in these creatures is imperfect, and the 

 blood is cold, or rather in place of having a fixed and inde- 

 pendent temperature, it tends without ceasing to an equi- 

 librium with that of the surrounding medium. 



It follows from this, that, in certain circumstances, the 

 temperature of animals with cold blood is considerably ele- 

 vated ; it is therefore better to call the mammalia, birds, 

 reptiles, and fishes, animals of a fixed and variable tempera- 

 ture with reference to the heat of their blood, instead of 

 warm and cold-blooded animals.* 



The course of the blood in man and the animals most 

 nearly allied to him in organization, appears to be as fol- 

 lows : The blood, charged with the restorative materials of 

 nutrition, is brought to each organ by the arteries and their 

 numerous ramifications. It spreads itself in the tissues of 

 these organs by a system of fiue anastomosing vessels called 

 capillary vessels. In these same organs arises another 

 order of vessels commencing where the arteries terminate ; 

 these tubes, at first capillary, communicate with the arteries, 

 and constitute by their re-union, the veins which carry back 

 the blood which has served to nourish each organ to the 

 heart. The blood therefore fiows in the veins in a oon- 

 trai-y direction to the current which circulates in the arte- 

 ries. These veins gradually unite among themselves, and 

 finally form two large trunks called the inferior and supe- 

 rior vena cava. These two vena caow meet in the cavity 

 of the right auricle, from whence the blood passes into a 



* See " Elements d'Histoire Naturelle Medioale," par Achille Rich- 

 ard. Tomo premier. Premiere partie. Zoologio, page 27. 



