THE BLOOD FLOW 401 



supposition in determinations of this kind is that the venous drainage balances the 

 arterial influx and that an increase or decrease in the volume of an organ may be 

 taken as a measure of its vascularity. This inference may be a safe one to make 

 when dealing with passive and compact organs, but may lead to errors if the part 

 experimented upon is soft in texture and embraces varying amounts of active 

 tissue elements. The plethysmograph has also been employed for the registration 

 of those changes in the volume of parts which occur in consequence of the activity 

 of the heart or respiration, and also in consequence of different experimental 

 procedures. In all these cases it is assumed that the alterations in the volume 

 of a part are dependent upon displacements of fluid and are therefore directly 

 attributable to changes in its blood supply. When a study is made of the volume- 

 curve of the arm it will be seen to be made up of smaller and larger oscillations, 

 the first of which occur synchronously with the action of the heart, and the second, 

 with the respiralory motions. This means that the systolic discharge of the heart 

 increases the vascularity of this part momentarily and that a similar increase 

 takes place throughout inspiration. A most striking demonstration of these 



Fig. 215. — Plethysmographic Curve of Forearm. 

 Showing the cardiac and respiratory variations in the volume of the arm. The 

 decided decrease in its volume observed here is due to mental activity; hence, to a 

 transfer of blood from the cutaneous circuits into that of the cerebrum. {Howell.) 



changes may be had by observing the surface of the brain through a rather small 

 trephine opening which contains a small quantity of v/armed saline solution. 

 The level of the solution will be seen to rise with every systole and to fluctuate in 

 larger waves with every respiration. 



When taken with a fairly sensitive apparatus., the general appearance of the 

 volume-curve of a part presents practically the same details as a tracing of the 

 blood pressure. It displays not only the cardiac and respiratory oscillations, but 

 also Traube-Hering waves and all those variations which are dependent upon more 

 lasting increases or decreases in the blood supply. In this way, for example, it has 

 been demonstrated by Mosso that the vascularity of the brain is diminished during 

 sleep, because the intracranial blood is transferred during this period into other 

 circuits of the body. 



The chemical method which has been introduced by Bornstein^ is founded upon 

 the principle that the volume of blood passing through the lungs of a man may 

 be obtained by calculation from the quantity of nitrogen absorbed by the blood. 

 This value is derived from the tension difference of this gas in the alveolar air and 

 the blood. Zuntz and his co-workers, ^ as well as Krogh and Lindhard,^ employed 



1 Pfliiger's Archiv, xxxii, 1900. 



2 Zeitschr. fiir Balneologie, iv, 1912. 



3 Skand. Archiv fiir Physiol., xxvii, 1912, 100. 

 26 



