298 CmCULATOHY APPARATUS, 



§ 262. The S-shapecl curvature of the cardiac tube advances 

 simultaneously with the partial obliteration of the aortic arches. 

 This it is which determines the position of the heart. For as 

 soon as it is complete, we find the lower loop of the S, the future 

 apex, turned towards the left side, we see the venous sinus enter- 

 ing below and behind from the right side, and the aorta given 

 off above and in front, towards the right, bending back to form 

 the arch with the second turn of the S (fig. 95). 



If we inquire into the mutual interconnexion of these pheno- 

 mena, we shall find it necessary to give an indirect and circuitous 

 answer. We must start from the proposition 

 Fig. 95. ^j^^^ ^ column of fluid, driven through an elas- 

 tic tube under high pressure, tends to assume 

 a spiral form. This is easily shown by fasten- 

 ing a relatively narrow tube of gutta-percha to 

 the nozzle of a water-tap, from which a full and 

 powerful stream is issuing. Even the simple 

 cylindrical column exhibits the rotation when 

 it is looked at, if possible, in front or from 

 behind ; but if wo compress the aperture of 

 exit slio'htlv, so as to mye it more of an oval 

 form, we may see the spiral curve of the flat- 

 The heart of the tened column from the side as well. Tlie ex- 

 embryo after the perirnent is reallv superfluous, since we per- 

 completion of its J^ . . " t • 



S - shaped flex- form it every time we pass water. It is only 

 ure. The trunks -^ylien the detrusor vcsiccc is too feeble to drive 

 the archln their ^^^^ urine through the urethra under the usual 

 final order. Due- pressure, that the spiral twist of the stream is 

 Botalli^^ eiiosus ^ff^^^^ . {^qi^qq its absence is justly regarded as 

 a premonitory symptom of paralysis of the 

 bladder. The above remarks apply with equal force to the 

 flow of the blood through the vessels. The column of blood is 

 spirally twisted, or, what comes to the same thing, it behaves 

 like a cylinder which has been spirally twisted in one direction ; 

 and it imparts this twist to the entire vessel whose form it 

 regulates. 



Whether a body is twisted fi'om right to left, or from left to 

 right, may be seen by approximating any two of its points which 

 are not too far apart, and so making it bend at an angle. 

 Suppose by way of illustration we take a pocket-handkerchief, 



