608 repokt— 1878. 



WEDNESDAY, AUGUST 21, 1878. 



The following Tapers were read : — 

 1. The Bate of Cardiac Hijp extrophy. By William H. Stone, M.A., F.B.C.P. 



It may be accepted as an axiom that every reduction of physiological or patho- 

 logical processes to known physical laws, even if only partially and approxi- 

 mately successful, is a step forward in science, and that if the whole disturbance of 

 Nature's equilibrium, from the excessive complexity of the functions involved, be 

 beyond our power of analysis, there is still some advantage and a gain to precise 

 observation in cutting off and putting on one side such portions as are susceptible 

 of accurate treatment. The predominance of vague, though necessary generaliza- 

 tions, such as that of vital action, is thereby materially diminished, and the direction 

 is more closely indicated in which future efforts at advance may most judiciously 

 be directed. 



Of all parts of the human organism, there is none, after the simple muscular and 

 bony framework, which belongs more distinctly to Physics than the apparatus of 

 circulation, and its central motor, the heart. This organ is obviously nothing more 

 than a double force-pump, furnished with two reservoirs and two pipes of outflow. 

 It is true that the valvular apparatus, which is also duplicated, is furnished with 

 complex muscular and tendinous appendages to regulate the tension and secure the 

 apposition of the occluding surfaces. But in the main the problem of its action is 

 hydrodynamical, and many valuable facts may be obtained by regarding it in a 

 mechanical point of view. The height to which this force-pump is called upon to 

 raise a o-iven volume of fluid is obviously measured by the height of the animal to 

 which it belongs, and even as early as 1769 Dr. Hales made experiments on horses, 

 which showed the height of the column of fluid sustained by the hearfs contraction, 

 as transmitted through the large arteries, to be in horses about 9-14- feet. 



Dr. Haughton, in his excellent and suggestive work on 'The Principles of 

 Animal Mechanics,' has most ingeniously utilized an accidental opportunity afforded 

 by an operation for computing the force when a large artery is divided in the 

 human subject, at a vertical column of 2-58 feet; that ascertained to exist in the 

 horse being 2-53 feet. It would, indeed, seem that the maximum hydrostatical force 

 is about the same in the two animals, and probably not far different in the sheep 

 and dog. 



Besides (1) the statical pressure of the column equal to the animal's height, 

 which has to be sustained, and (2) the force consumed in overcoming the inertia of 

 the blood-mass, an important element in the work to be done is (3) the resistance 

 offered by the capillary vessels to the flow of blood. There is reason to believe, 

 says Dr. Haughton, that in animals similar in bulk the arrangement and 'structure 

 of the capillaries are such that the ratio of the squares of their cross-sections to 

 their total length is practically constant. Supposing the co-efficient of capillary 

 resistance to be the same in the man as in the horse, we can calculate, on Dr. 

 Haughton's method, the hsemastatical pressure of blood in the human arteries. 

 The left ventricle has a capacity of about 3 ozs., or 5-2 cubic inches, and beats about 

 75 times per minute. 



Taking the usual equation of capillarity — 



Q = A*_- 



Where Q = quantity discharged in a given time. 

 A = a constant. 



h = the charge, or hydrostatical pressure. 

 d = diameter. I = length of tube. 



