THE VASOMOTOR NERVES. 595 



any organ is accompanied by the reverse changes. Peripheral 

 resistance being diminished there will be a fall of pressure on the 

 arterial side and a rise of pressure on the venous side. When, 

 therefore, the stimulation of any nerve brings about a rise of 

 arterial pressure that can not be referred to a change in the heart 

 beat the inference made is that the result is due to a vasocon- 

 striction. When the method is applied to a definite organ — the 

 brain, for instance — it becomes conclusive only when simultaneous 

 observations are made upon the pressure in the artery and the vein 

 of the organ, and proof is obtained that the pressures at these points 

 vary in opposite directions. 4. By observations upon the volume 

 of the organ. It is obvious that, other conditions remaining un- 

 changed, a vasoconstriction in an organ will be accompanied by 

 a diminution in volume, and a vasodilatation by an increase in 

 volume. This method of studying the blood-supply of an organ is 

 designa.ted as plethysmography, and any instrument designed to 

 record the changes in volume of an organ is a plethi/smograph* 

 Plethysmographs have been designed for special organs, and in such 

 cases they have sometimes been given special names. Thus, the 

 plethysmograph used upon the kidney and spleen has been design 

 nated as an oncometer, that for the heart, as a cardiometer. 

 The precise form and structure of a plethysmograph varies, of 

 course, with the organ studied, but the principle used is the 

 same in all cases. The organ is inclosed in a box with rigid 

 walls that have an opening at some one point only, and this 

 opening is placed in connection with a recorder of some kind by 

 tubing with rigid walls. The connections between recorder 

 and plethysmograph and the space in the interior of the latter 

 not occupied by the organ may be filled with air or, as is more 

 usually the case, with water. The idea of a plethysmograph 

 may be illustrated by the skull. This structure forms a natural 

 pelthysmograph for the brain. If a hole is bored through the 

 skull at any point and a connection is then made with a recorder 

 of some kind, such as a tambour, the volume changes of the 

 brain may be registered successfully. 



The plethysmograph generally employed in laboratories, particularly for in- 

 vestigations on man, is some modification of the form devised by Mosso (see 

 Fig. 249) . The hand and more or less of the arm is placed in a glass cylinder 

 which is swung freely from a support. The opening around the arm is shut 

 off by a cuff of rubber dam that must be chosen of such a size as to 

 fit the arm snugly without compression of the superficial veins. The 

 forward end of the plethysmograph is connected by tubing with a re- 

 corder. Through appropriate openings the cylinder and connecting tubes 

 are filled with warm water and then all openings are closed except the 

 one leading to the recorder. Any increase in volume of the arm will drive 

 water from the plethysmograph to the recorder, and any decrease, on 



* For a description of the development of this method, see Fran<;ois-Franck- 

 Marey's " Travaux du Laboratoire, " 1876, p. 1. 



