104 GENERAL PRINCIPLES OF ZOOLOGY 



cannot re-enter the blood-vessels in the same way, on account of the 

 higher pressure in the capillaries. This overflow is conducted back to 

 the veins by the lymph-vessels. These begin with lacunae in 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 chyle-vessels, because they contain the chyle, dis- 

 tinguished from ordinary lymph by its milky color. 



Cold- and Warm-blooded Animals. In connection with the blood- 

 vascular system, two expressions are much used but not generally correctly 

 understood, viz., cold-blooded and warm-blooded or, more correctly, 

 animals with variable and animals with constant temperatures. Under the 

 head of animals with varying temperature (poikilothermal} or cold blood 

 are placed forms whose temperature is largely dependent upon the tem- 

 perature of the- environment, rising and falling with it, but usually a few 

 degrees above it. In our climate, where the atmospheric temperature is 

 considerably lower than the temperature of the human body , such animals, 

 for example the frog, feel cold to our touch, since they have a much lower 

 temperature than we. 



Such creatures as maintain about the same temperature, under 

 any thermal condition are termed warm-blooded or constant temperatured, 

 (idiothermal, homoiothermal) animals. Man in summer and winter 

 under the equator and at the north pole, has approximately a temperature 

 of 36 C. (98! F.), showing higher temperatures only in fever. In order 

 to maintain a constant temperature during the varying conditions, the 

 animal must have the power to regulate the warmth of its body, either 

 by limiting the production of heat, or by controlling its loss. If the en- 

 vironment be warmer than is suitable for the body temperature, then the 

 production of heat must be limited to the smallest quantity compatible 

 with the vital processes; but, if this does not suffice, the loss of heat must 

 be increased by evaporation from the surface, usually accomplished by 

 active perspiration. If, on the contrary, the environment be cold, then 

 every unnecessary loss of heat must be avoided, while the production of 

 heat must be increased. It is clear that idiothermy, since it requires 

 complicated apparatus, can occur only in the highly organized animals. 



IV. Excretory Organs. 



The excretory organs are tubes or glandular canals which open to the 

 exterior either directly or by way of an end-gut (cloaca), and conduct 

 substances which have become useless to the body to the outside. The 

 presence of a blood-vascular system or a ccelom or both exercises an 



