FUNCTIONS OF BLOOD I9I 



have to be carried out somehow even if the animal had no blood 

 system ; it is difficult to imagine that any of them, with the 

 partial exception of oxygen transport, which in small animals 

 may be done by a system of air-containing tubes, could be 

 properly carried out except by a circulating liquid. Various types 

 of animal have also taken advantage of the existence of blood 

 to use it for other and minor purposes, which can equally well 

 be fulfilled in other ways. It is an accident that the oxygen- 

 carrying substances contained in most bloods are pigments, and 

 in a number of animals they are visible from the outside, possibly 

 or probably to the advantage of the species. The most familiar 

 example is man himself, for in the white races the colour of the 

 haemoglobin is presumably important in sexual selection, but 

 its most striking use is seen in some monkeys, where the chest 

 or buttocks blush vividly in excitement. As is well known, a 

 fluid can transmit a pressure and so be used to increase a volume, 

 and these properties have more than once been used by animals. 

 The swan mussel moves by pushing its foot forward through 

 the mud and then inflating it with blood. When the muscles 

 which withdraw the foot are contracted, the latter is stuck 

 fast, and so the whole animal is pulled forward ; the blood is 

 then withdrawn and the process repeated. The swelling of the 

 labium of blood-sucking insects, the eversion of the insect- 

 catching tongue of the chamaeleon, and the increase in size 

 and stiffness of the mammalian penis necessary for copulation, 

 are brought about in a similar way. The method used in all 

 these instances is largely the same ; the veins are constricted 

 so that the blood which is pumped in by the heart can only escape 

 slowly and is retained under pressure. The resulting turgidity 

 is comparable to that w^hich is normal in herbaceous plants, 

 although here the fluid is enclosed and its pressure is osmotic, 

 not hydrostatic, in origin. In both the Amphibia and the insects 

 the phagocytes in the blood assist in metamorphosis by ingesting 

 unwanted material, such as the tadpole's tail, and transferring 

 it elsewhere. Finally, in mammals and birds, the blood-vascular 

 system, rather than the blood itself, is used in temperature 

 regulation. Although the blood takes no direct part in temperature 

 control, its presence and circulation are necessary to the efficient 

 working of such control, just as a stirrer increases the efficiency 

 of a thermostat. In large animals without temperature regulation 

 the blood is probably important in preventing local rises of 



