454 THE RABBIT. PHYLUM CHORDATA 



osmotic pressure of the proteins in the blood, but most of it is 

 collected into lymphatic vessels. Those from most of the body 

 join together and open into the left subclavian vein at its junction 

 with the external jugular, but those of the right side of the head 

 and neck and right fore-hmb open into the right subclavian at 

 the corresponding point. The lymphatic system of man, which 

 is substantially similar to that of the rabbit, is shown in Fig. 358. 



It is obvious from its almost universal occurrence in the higher 

 animals that blood must be important in their hfe. It has many 

 functions, and those are seen at their greatest number in mammals. 

 A fuller account of its functions is given on p. 190, and some details 

 of its physiology will be found elsewhere in this book, but it is 

 convenient to hst its chief functions here. It is primarily a means 

 of transport, and the things which it carries are five : oxygen in 

 the red cells (p. 524) ; food in solution (p. 444) ; excretory 

 products including carbon dioxide in solution, and carbon dioxide 

 in the red cells ; hormones (p. 470) ; and heat. The con- 

 vection of heat by the blood is important in all large animals, 

 irrespective of whether they are warm or cold-blooded, as without 

 it the active tissues, such as muscles, would become over-heated. 

 Secondly, the blood, in conjunction with other organs, regulates 

 its own composition, so as to provide a constant internal environ- 

 ment for the cells of the body ; in particular it maintains a con- 

 stant hydrogen ion concentration, and, in conjunction with 

 the kidney, constant proportions of water, glucose, and other 

 substances (p. 379). Thirdly, it is used for a number of 

 miscellaneous minor purposes ; in some mammals, such as 

 man and notably many monkeys, it imparts colour to the 

 body, and it is used mechanically for the erection of the penis. 



In mammals and birds the blood vascular system is also 

 important in temperature regulation. In these warm-blooded 

 or homoiothermic animals the temperature of the body is kept 

 constant within very narrow limits, whatever the temperature 

 of the surroundings. The usual temperature of mammals is about 

 38° C. Control is obtained in two ways : as the temperature 

 outside falls, the amount of heat produced by the resting muscles 

 increases, so that more is available to be lost to the environment, 

 and at the same time the surface capillaries of the body contract 

 so that less blood flows through the skin and less heat is brought 

 to the surface to be lost. The result is that the temperature of 

 the skin falls but not that of the rest of the body. The reverse 



