496 Prof. W. A. Osborne and Mr. W. Sutherland. [July 5, 



some time be allowed to elapse at each stage before reading the pressure, as 

 this always tends to rise somewhat. When the elastic limit of a bladder is 

 reached, the gradient of the pressure rise is very steep and the rise is not 

 a linear function of the volume. 



There is always a danger that in investigations on elasticity one may forget 

 that the viscus in question in the living animal is supplied with reactive 

 muscle, and that only when this muscle is fully inhibited can the pure 

 physical elasticity of the walls play a predominating part. It is a mistake to 

 describe the flow of blood in the systemic arteries as a flow of liquid in elastic 

 tubes. Such is certainly the case in the aorta, and possibly in the larger 

 arteries, but in the arterioles and smaller arteries only when the muscle is fully 

 inhibited or killed. To describe the circulation as occurring through a system 

 of muscular tubes, with some elastic tissue aiding the muscles, would be more 

 accurate. Similarly with the bladder and other hollow viscera (except the 

 lung), the elastic tissue acts merely as an adjuvant to the muscle, economising 

 the work of the latter ; but it is the muscle which plays the preponderating 

 part in determining the tension of the visceral wall. 



Conclusions. 



1. When initial rigidity is present in a rubber balloon, the pressure on 

 inflation rises rapidly at first, then falls, and tends to remain at a constant 

 value until the elastic limit is reached. 



2. Such a balloon on deflation displays a marked hysteresis. Only rarely 

 will the pressure rise on deflation. 



3. If initial rigidity be abolished by keeping a balloon inflated some time 

 and then rapidly deflating, the pressure on a new inflation rises consistently. 

 On plotting pressure against radius in such cases a rectangular hyperbola may 

 be obtained, satisfying the equation 



(r — a) (p — b) = c, 



where a is close in order of magnitude to the initial radius, and & is a 

 constant greater than p. The behaviour of such a balloon is, however, 

 far removed from that of a sphere of perfectly elastic and isotropic 

 material. 



4. When the elastic limit is reached in a rubber balloon the pressure is a 

 linear function of the volume. 



5. Hollow viscera approximately spherical, such as the bladder, do not 

 display initial rigidity, and never give a fall of pressure with increasing 

 volume. When the elastic limit is reached, the pressure is not a linear 

 function of the volume. 



