490 A PARENTHETIC NOTE [ch. 



Lastly, a very instructive case is furnished by the arrangement 

 of the muscular fibres on the surface of a hollow organ, such as 

 the heart or the stomach. Here we may consider the phenomenon 

 from the point of view of mechanical efficiency, as well as from 

 that of purely descriptive or objective anatomy. In fact we have 

 an a 'priori right to expect that the muscular fibres covering such 

 hollow or tubular organs will coincide with geodetic lines, in the 

 sense in which we are now using the term. For if we imagine a 

 contractile fibre, or elastic band, to be fixed by its two ends upon 

 a curved surface, it is obvious that its first effort of contraction 

 will tend to expend itself in accommodating the band to the 

 form of the surface, in "stretching it tight," or in other words 

 in causing it to assume a direction which is the shortest possible 

 line wpon the surface between the two extremes : and it is only 

 then that further contraction will have the effect of constricting 

 the tube and so exercising pressure on its contents. Thus the 

 muscular fibres, as they wind over the curved surface of an organ, 

 arrange themselves automatically in geodesic curves : in precisely 

 the same manner as we also automatically construct complex 

 systems of geodesies whenever we wind a ball of wool or a spindle 

 of tow, or when the skilful surgeon bandages a limb. In these 

 latter cases we see the production of those " figures-of-eight," to 

 which, in the case for instance of the heart-muscles, Pettigrew 

 and other anatomists have ascribed pecuhar importance. In the 

 case of both heart and stomach we must look upon these organs 

 as developed from a simple cylindrical tube, after the fashion of 

 the glass-blower, as is further discussed on p. 737 of this book, 

 the modification of the simple cyhnder consisting of various degrees 

 of dilatation and of twisting. In the primitive undistorted 

 cyhnder, as in an artery or in the intestine, the muscular fibres 

 run in geodetic lines, which as a rule are not spiral, but are merely 

 either annular or longitudinal ; these are the ordinary " circular 

 and longitudinal coats," which form the normal musculature of 

 all tubular organs, or of the body- wall of a cyUndrical worm*. If 

 we consider each muscular fibre as an elastic strand, imbedded in 

 the elastic membrane which constitutes the wall of the organ, it 



* However, we can often recognise, in a small artery for instance, that the so- 

 called "circular" fibres tend to take a slightly oblique, or spiral, course. 



