490 



SCIENCE 



[N. S. Vol. XXV. No. 639 



like the organs for internal secretion, seem 

 to be provided with a larger surplus of 

 tissue. 



The complex apparatus of circulation is 

 well provided with factors of safety. In 

 the first place, the animal body possesses a 

 good deal more blood than it requires for 

 its work. It is known by experimental 

 evidence and clinical observations that 

 nearly one half of the blood can be with- 

 drawn without serious consequences to the 

 life of the animal. As a further factor of 

 safety in this regard we might register the 

 ability of the blood to recover its loss very 

 rapidly. 



Furthermore, the capacity of the entire 

 system of blood vessels in a completely re- 

 laxed state is again much greater than the 

 volume of blood of the body. It is this 

 difference between the volume of blood and 

 the volume of the vessels which greatly 

 facilitates the circulation of the blood and 

 the proper nutrition of the various organs 

 of the body. On the basis of this differ- 

 ence large quantities of blood can be 

 thrown at once and with ease into the 

 splanchnic region, into the skin or into the 

 working muscles. After a local injury or 

 infection in a very brief time for the sake 

 of repair or defence hypersemia sets in, 

 and vessels which were not noticeable be- 

 fore become fairly visible. An instance 

 of a similar order is the wide-spread insti- 

 tution of collateral circulation. Around 

 an anemic focus blood vessels which pre- 

 viously were hardly visible become full and 

 large to meet the threatening danger of 

 necrosis of the neighboring anemic tissues. 

 All these devices which spring into activity 

 only under special exigencies, are mani- 

 festly factors of safety and are made pos- 

 sible by a superabundance of bloodvessels. 



The difference between blood volume and 

 capacity of vessels is an indispensable 

 factor of the circulation, and its per- 



manence is assured by many devices. 

 Thus, for instance, any artificial increase 

 of the volume of blood is immediately cor- 

 rected through the chief eliminating or- 

 gans, or through the secretory glands, or 

 even by throwing some of the surplus 

 serous fluid temporarily into the lymph 

 spaces and serous cavities. OEdema, as- 

 cites and hydrothorax are sometimes not 

 parts of the affliction, but means of repair. 



Furthermore, existence of the difference 

 between vascular capacity and quantity of 

 blood is made possible only by a wonderful 

 mechanism which controls in every part of 

 the body the mutual adaptation of blood 

 and vessel— the so-called vasomotor appa- 

 ratus. It causes the dilatation of the ves- 

 sels in the part of the body which requires 

 and is to receive more blood, at the same 

 time causing a constriction of the vessels 

 in a part which can spare some of its 

 blood. This mechanism is so important 

 that it is again guarded by an abundance 

 of factors to assure its safety. There is 

 a vasomotor center in the mediiUa ob- 

 longata; when this is destroyed, a number 

 of vasomotor centers in the dorsal medulla 

 assume control; when they are eliminated, 

 the sympathetic ganglia take over the com- 

 mand, and when they too drop out, the 

 vascular wall itself attends to the proper 

 regulation and adaptation of the capacity 

 of the vessels to the volume of blood. 



Finally, the chief motor mechanism of 

 the circulation, the heart, is a clear in- 

 stance of an organ provided with a super- 

 abundance of volume and force. Normally 

 it is in a state of tonus and receives only 

 a moderate volume of blood which it 

 throws into the aorta with no great hurry 

 and with an expenditure of only a moder- 

 ate amount of energy. But at any mo- 

 ment it is ready to receive many times the 

 usual volume of blood, is ready to double 

 or treble the rate of its beats and is capable 

 of developing nearly any amount of energy 



