GEXERAL ORGANOLOGY. 137 



occurs only in vertebrates. In the capillary region of the 

 body, it is true, proteids may pass over into the tissues, 

 but it is self-evident that a possible overflow cannot re- 

 enter the blood-vessels in the same way, on account of the 

 higher pressure prevailing in the capillaries. This over- 

 flow is conducted back to the veins through the lymph- 

 vessels. The lymph-vessels begin with the lacuna; of the 

 tissues, and gradually pass into vessels with definite walls. 

 The lymph-vessels of the digestive tract are particularly 

 important since, during digestion, they become filled with 

 the proteid and fatty constituents of the digested food ; 

 they are called the chyle-vessels, because their contents, 

 the chyle, is distinguished from ordinary lymph by its 

 milky color. 



Cold- and Warm-blooded Animals. In connection 

 with the blood-vascular system, two expressions which are 

 much used but not generally correctly understood by the 

 general public, viz., cold-blooded and warm-blooded or, 

 more correctly, animals with variable and animals with defin- 

 ite temperatures should be explained. Under the head of 

 ?.iimals with varying temperature (poikilothermous) or cold 

 blood are placed forms whose temperature is entirely 

 dependent upon the temperature of the environment, 

 rising and falling with it, but usually a few degrees above 

 it. In our climate, where the atmosperic temperature 

 is considerably lower than the temperature of the human 

 body, such animals, for example \\\c frog, would feel cold 

 to our touch, since they, particularly in the cool season, 

 have a much lower temperature than we. 



Such creatures as, living under any thermal condition, 

 maintain about the same temperature, are termed warm- 

 blooded or definite-temperatured (idiothermous, homoio- 

 thermous) animals. Man in summer and winter, under the 

 equator and at the north pole, has approximately a tem- 

 perature of 36 C., showing higher temperatures only in 

 fever. In order to maintain a constant temperature dur- 

 ing the varying external thermal conditions, the animal 

 must have a heat-regulator; it must have the power to 



