536 



THE SUBINTESTINAL VEIN. 



,sp.c 



really rhythmically contractile 1 tlie fact would be interesting as shewing 

 that this quality, which is now localised in the he.-irt, was once probably 

 common to the snbintestinal vessel for its whole length. 



On the development of the cardinal veins (to be described below) 



considerable changes are effected in the 

 subintestinal vein. Its postaual section, 

 which is known in the adult as the caudal 

 vein, unites with the cardinal veins. On 

 this junction being effected retrogressive 

 changes take place in the prteanal section 

 of the original subintestinal vessel. It 

 bieaks up in front into a number of 

 smaller vessels, the most important of 

 which is a special vein, which lies in the 

 fold of the spiral valve, and which is 

 more conspicuous in some Elasmobranchii 

 than in Scyllium, in which the develop- 

 ment of the vessel has been mainly 

 studied. The lesser of the two branches 

 connecting it round the cloaca with the 

 caudal vein first vanishes, and then the 

 larger; and the two posterior cardinals 

 are left as the i-ole forward continuations 

 of the caudal vein. The latter then be- 

 comes prolonged forwards, so that the 

 two cardinals open into it some little 

 distance in front of the hind end of the 

 kidneys. By these changes, and by the 

 disappearance of the postanal section of 

 the gut, the caudal vein is made to 

 appear as a supraintestinal and not, as 

 it really is, a subintestinal vessel. 



From the subintestinal vein there is 

 given off a branch which supplies the 

 yolk-sack. This leaves the subintestinal 

 vein close to the liver. The liver, on 



its development, embraces the subintestinal vein, which then breaks 

 up into a capillary system in the liver, the main part of its blood 

 coining at this period from the yolk-sack. 



The portal system is thus established from the subintestinal vein; 

 but is eventually joined by the various visceral, and sometimes by the 

 genital, veins as they become successively developed. 



The blood from the liver is brought back to the sinus veuosus by 

 veins known as the hepatic veins, which, like the hepatic capillary 

 system, are derivatives of the subintestinal vessel. 



FIG. 3;>7. SECTION THROUGH 

 THE TRUNK OF A ScYLLIUM KMJiKYO 

 SLIGHTLY YOUNGER THAN 28 F. 



np.c. spinal canal ; If". v,hite 

 matter of spinal cord ; ]. poste-- 

 lior nerve-roots ; rh, notochonl ; 

 .c. sub-notochol'daliod; ao. aorta; 

 hip. muscle-plate; -nip', inner layer 

 of muscle-plate already converted 

 into muscles; 7>. rudiment of 

 vertebial body; *f. segmental 

 tube; *</. segrnental duct; xp.r. 

 spiral valve ; r. subintestinal vein ; 

 /.. primitive generative cells. 



.1. duller holds that this sack is not rhythmically contractile. 



