684 SPECIAL PHYSIOLOGY. 



of blood within the cranium must either be always the- same, or else 

 some special provision must exist for its increase or diminution in quan- 

 tity. It has been suggested that the quantity of blood in the cranium 

 is absolutely unalterable, and that the only changes which can take 

 place in the cerebral circulation, are various compensatory displace- 

 ments of the blood in the interior of the arteries, veins, and capil- 

 laries ; but experiments have shown that the brain of an animal may 

 be rendered pallid, i. e., may be deprived of the blood in its vessels 

 by extreme venesection. Moreover, the presence of the cerebro-spinal 

 fluid (p. 235), and the known rapidity with which the secretion and ab- 

 sorption of so diffluent a fluid may take place, afford a feasible explana- 

 tion of the mode in which variations in the quantity of blood in the 

 vessels within the cranium may be rapidly counterbalanced. 



The pulmonary circulation presents many peculiarities. Its arteries 

 convey dark or deoxygenated, and its veins bright or oxygenated, 

 blood. Neither its veins or arteries anastomose, except in their very 

 finest ramifications; its veins have no valves, either in their course, or 

 at their entrance into the left auricle ; its capillaries are large, most 

 numerous, and very short between the arteries and veins. As every 

 part of the pulmonary circulation is carried on within the thorax, the 

 flow of blood from the right ventricles, through the pulmonary vessels, 

 to the left auricle, is, unlike the systemic circulation, equally influenced 

 in every part, at each moment, by the varying conditions of thoracic 

 pressure. Lastly, the loops of the pulmonary circulation are much 

 shorter than those of the systemic vessels, and the blood takes much 

 less time in passing through them. The velocity of the blood is 

 greater, and the blood pressure much less. 



Period of a Complete Circulation. 



It hag been seen that the chief cause of the circulation of the blood 

 in Man, and in animals possessing a heart, is undoubtedly the muscu- 

 lar force of that organ ; that the relative velocity of the blood-current 

 in its several parts, is quickest in the arteries, slower in the veins, and 

 slowest, by many degrees, in the capillaries, the actual rate in the 

 large arteries being about 10 inches per second, in the small arteries 

 probably about 2.2 inches per second, in the capillaries about g'^th of an 

 inch per second, and in the medium-sized veins from about J to Jd of 

 the rate in the corresponding arteries ; and lastly, that the rate of 

 movement through the pulmonary circulation is five times more rapid 

 than that through the systemic circulation. There remains yet to in- 

 quire, in what period of time the complete circulation is performed, 

 that is to say, in what time, a given minute portion of blood, thrown 

 from the left ventricle, or passing any other given point of the circu- 

 lation, will flow through the body and lungs, back to the same point. 

 The conclusion arrived at on this subject, based on many experiments 

 in animals, is, that, in Man, a complete circulation of a given particle 

 of the blood may be performed within a much less time than one min- 

 ute. A solution of ferrocyanide of potassium (selected for the facility 

 with which it ma^ be detected by appropriate chemical tests), being 



