BY DR. BURDON-SANDERSON. 227 



the less will be the effect ; so that the condition of an artery 

 in which the expansive movement is relatively greatest, is that 

 in which its walls, when the expanding agency is suspended. 

 are in the state of elastic equilibrium, i.e., when the minimum 

 pressure is least. A moment's consideration teaches us that 

 there are two circumstances which must diminish the minimum 

 pressure in the arteries, viz., diminution of the mean arterial 

 pressure, and prolongation of the period which intervenes be- 

 tween one expansive act and its successor. In other words, 

 the less frequent the contractions of the heart and the lower the 

 arterial pressure, the greater the expansion in proportion to 

 the. expanding force which produces it. 



38. The Sphygmograph. In man, no artery can be di- 

 rectly measured either as regards pressure or expansion. In 

 feeling the pulse, we attempt to measure both by the sense of 

 touch, and obtain results, which, although incapable of nu- 

 merical expression, are sufficiently exact to be of great value. 

 In the sphygmograph, an attempt has been made to obtain the 

 same kind of information by a mechanical contrivance, which 

 the physician obtains by the tactus eruditus ; the supposed 

 advantage of the instrumental results over the others being, 

 that they can be estimated by measurement and weighing, and 

 that they are unaffected by variation in the skill and tactile 

 sensibility of the observer. 



The purpose of the sphygmograph is to measure the com- 

 plicated succession of alternate enlargements and diminutions 

 which an artery undergoes whenever blood is forced into it 

 by the contracting heart, to magnify those movements, and to 

 write them on a surface, progressing at a uniform rate by 

 watch-work. 



The construction of the instrument is so well known, that 

 it is scarcely necessary to give a detailed description of it. It 

 consists essentially of three parts: a frame of brass which is 

 applied along the outer edge of the volar aspect of the fore- 

 arm, in such a way that it is maintained in a fixed position 

 with reference to the bones of the wrist and radius a steel 

 spring which, when the instrument is in use, presses upon the 

 radial artery and receives its movements and lastly, mechani- 

 cal arrangements for maguifj'ing these movements and record- 

 ing them. Both of these ends are accomplished by means of a 

 light wooden lever (A A', fig. 208) of the third order, which is 

 supported by steel points (c). There is a second lever of the 

 same order (B E) which has its centre of movement near the 

 attachment of the spring (at E). It terminates in a vertical 

 knife-edge (D), and is traversed by a vertical screw (T). When 

 the extremity of the screw (N) rests upon the spring above the 

 ivory plate, every movement of the plate is transmitted to this 

 lever (B E), and, by means of the knife edge, to the wooden 



