182 THE STRUCTURE OF MAN 



blood from the aorta to the placenta. The basal portions of these 

 vessels often remain patent throughout life, and function as 

 superior vesical arteries ; the remainder of each, however, i.e. by 

 far its greater portion, loses its lumen altogether and becomes 

 a solid strand of connective tissue. 



[Considerable interest attaches to those veins of the very 

 variable " vesico-prostatic plexus " which, in the adult, in proxi- 

 mity to the above-named arteries, carry back the blood from the 

 urinary bladder to the internal iliac veins. The detailed re- 

 lationships of certain varieties of these would seem to suggest, 

 by analogy to the lower vertebrata, that they may be associ- 

 ated with the " anterior abdominal " venous system regularly 

 present in Birds, Reptiles, and Amphibians, and represented by 

 at least its main trunk, in the Monotreme Echidna 1 among 

 Mammals.] 



The continuation proper of the axis of the human aorta 

 is represented by a weak vestigial vessel, of very variable 

 relationships 2 the arteria sacralis media. In long-tailed 

 animals, in which the posterior end of the body has not 

 undergone reduction, this vessel is represented by the caudal 

 artery, which is a direct, gradually diminishing, continuation of 

 the aorta, originally giving off, like it, segmentally recurrent 

 branches. 



When we consider the polymeric origin of the limbs (cf. ante, 

 p. 67) dating back to an originally segmented condition of the 

 trunk, it is evident that their principal arteries must have arisen 

 in relation to segmental arteries of the body wall, and that 

 originally they in no way differed from these. This assumption 

 finds actual proof in the mode of origin of the arteria subclavia ; 

 but while it is comparatively easy to prove this for the fore- 

 limb, in the hind-limb a difficulty presents itself, since its corre- 

 sponding vessel at a very early period undergoes a great increase 

 in size and marked specialisation in relation to the development 

 of the umbilical artery. 3 In any case it is certain that the 



1 [Cf. Fenwick, Jour. Anal. andPhys., vol. xix. p. 320 ; and Beddard, Proc. Zool. 

 Soc., Lond., 1884, p. 553.] 



2 [These have been recently tabulated for 400 autopsies worked out by collective 

 investigation in medical schools, under the auspices of the Anatomical Society of 

 Great Britain and Ireland. In one instance the vessel appears to have been entirely 

 absent, cf. Jour. Anat. and Phys., vol. xxvii. pp. 184-187.] 



3 I cannot here enter further either into the question of primary origin, direct 

 from the aorta, of the arteria umbilicalis, or into that of the secondary connection 

 between this vessel and the arteries of the limbs. It must suffice to refer the reader 



