743 



one, then the trunk, becoming more important, acquires a flexion of its 

 own distinct from that of the tail, which latter organ often quickly be- 

 comes of minor imjjortance and diminishes in bulk. In this distinction 

 between Ciieiropicrmjia and Jc/tl///jop(frij(jfa lies the exi)lanation of the 

 production of the post-caval vein. So long as the trunk and the tail 

 constitute one continuous locomotor body, so long is there little chance 

 of the two posterior cardinals approaching the median line and com- 

 pletely fusing at a point midway in the length of the trunk, since this 

 point is necessarily anterior as regards the body as a whole, and anteri- 

 orly, flexion is of small degree 2. But on the development of locomotor 

 limbs, which act as fulcra in the movements of the body, flexion of the 

 trunk portion of the animal body becomes altered in character and, 

 instead of merely being, as it were a faint reflection of the violent move- 

 ments of the tail, becomes strongly accentuated and also acquires a new 

 stribution. In thedi trunk of a mammal ^ e. g. observation of the animal's 

 movements shows us that flexion of the trunk is greatest about 

 midway in its length, and decreases posteriorly towards the 

 1» el vis where flexion altogether ceases, as shown by vertebral fusion. 

 This being the case, the primitive posterior cardinals are subject to a 

 new distribution of forces, and, tending as in tishes to converge in the 

 region of greatest flexion, in consequence are replaced midway in the 

 length of the trunk by a median vein — the post-caval. 



At the same time, owing to the decrease of flexion towards the 

 anus (shown by the fusion of the sacral vertebrae) the posterior cardinals 

 are more at liberty to maintain their initial separate condition posteri- 

 orly, and, as a matter of fact, persist as the renal-portal veins in the 

 majority of Amniota, converging again, however, immediately behind the 



2 The posterior cardinals do to some extent meet and fuse anteriorly in the 

 ti'unk ot Elasmobranchs and so anteriorly as to form a cardinal sinus between the 

 gonads. But then the extreme activity and flexibility of these tishes must be borne 

 in mind, also the fact that the veins are large sinuses. It is significant that in these 

 fishes thus characterized, an anastomosis is formed between the cardinal sinus (or 

 post-caval) and the right hepatic sinus. See further in text. 



3 The occurrence of lateral fiexion of the trunk in higher vertebrata, besides 

 being obvious from actual observation , is also evidenced by the structure of the 

 trunk — by the separate condition of the vertebrae, the existence of ribs and the ar- 

 rangement of the trunk muscles, etc. ^Foreover, the existence of this flexion is evi- 

 denced by its efiect on internal organs like the liver, which, as Owen says. "is. as a 

 ride, divided into a greater number of lobes in the present [Mammalia^ than in the 

 preceding classes, the body being more flex nous at the seat of this viscus. 

 In the stift-trunked whales and erectly-mo\àng man the organ is more compact, and 

 it is least sub-divided in the inu-ely herbivorous Ungulates, where a minor degree of 

 hydrocarbonates has to be elimintated'", this organ being here very small. "When 

 flexion of the vertebral column does not occur, as e.g. in the sacral region of most 

 vertebrates above fishes, and in the region of the carapace of Chelonians, then fusion 

 of the constituents takes place. In anurous Amphibia the tadpole larval stage* 

 doubtless also forms a factor in the adult disposition of the vascular channels, and 

 it is of course possible that the median disposition found in all higher vertebrata is 

 in part a heritage from piscine ancestors. 



