] 906.] 



REXAL-J'ORTAI. SVSTIIM. 



897 



blood from tho iiiiid liDihs iind tji,il in tl)(j Miu]-dorsal cliannel, bvit 

 no fidl explfiiiation hiis yet Iteeu offered to account for the fact 

 that in the majority of the Jclithyopsida, and 8au]T)]),sid:i,, part of 

 the venous blood derived from the jiforenamed regions ;i,dopts a 

 mid-ventral coui'se in its pfissiige to the heart. What is the 

 exj)lanation of tlie ))roduction of j^elvic veins ? We cannot suppose 

 that the blood from tlie limlis is tlius in part deviated from the 

 primitive mid-dorsal course merely in f)rdertopass into tlie poi'tal 

 system of the liver, since the absence of any such feature in most 

 Fishes and in all Mamma,lia effectujilly negatives any such idea*. 

 Owen indeed elaborated an ingenuous tlujugh somewhat fragile 

 hypothesis which supported this idea, Ijut, as he liimself admitted, 

 it was of little value and solely apj)ertainetl to the coccygeo- 

 mesenteric of bii-ds and not indeed always to that t. 



In my opinion, a possiVjle sohition to this problem lies in the 

 fa,ct that the adoption of a. mid-vonitral way of i-etimi by a large 

 j)roportion of the venous ?;]ood is in all cases (save, and for the 

 reason below stated, Chelonia, and |if)-ha|)S Apteryx and other 

 llatit;e, in which there exists little or no mesfiwork) corj'elated 



* 0\V(.'ii rciiiiivks in cDiiiioctioi) vvitli tljis ijidiit that ''tlic iiiiiistoiiio.sis f)t' tlu; jjolvic 

 v<.'iiis, ill lieiii;^^ tin; iiiciiiis nf (•(iiivcyiiif;' cddiukih vcuous Idooil into tho liver, {^•rifs to 

 lirove tliat tlu; lilooil of tlie vcnn' jiorta; does not n-tjuire any jjeculiar preparation 

 liy circulation in tlie Kjileeii or otlicr viscera to fit it tor tlie secretion of bile." If 

 my theory is correct, it would seuiii tliat this is no proof. 



t Since this ingenuous theory of Owen is, so far as I know, the only one hitherto 

 ])ro])ounded to account for the jtrescnce of epigastric veins in Vertelirata, it seems 

 worth vvliile quotintr him fully on the suhject. " 'I'he venous sj'stem of the kidnej's 

 is so arrang:cd in hirds that tli<! hlood can he distributed either to the ])ortal system 

 hy the mesenteric vein, or to the pulmonary system hy the vena cava and rij^ht side 

 of the heart, according to the do;;ree of rapidity with whicli the i>ulinoiiary or portal 

 systems of veins are respectively sujijdied, or in other words, accoidinf; to the 

 activity with which the circulation in each of these systems may he ^^oiiij^ on at two 



diH'crent periods This disposition has been erroneously su]jp(jsed to indicate 



tiiat the urine was secreted from the venous hlood in hirds, as in reptiles and fishes ; 

 hut the end attained hj- the venous anastomoses in question bears a much closer 

 relation to the peculiar necessities and habit of life of the bird, and, so far as I know 

 lias not hitherto been cxidaimd. 'i'liere is no class of animals in which there may he, 

 at anj' two bi'ief and consecutive periods of existence, a greater difference in the 

 degree of energy and rajjidity with which the respiratory functions are performed 

 than in hirds. When the bird of prej', for exainph;, stimulated by a hungry and 

 an (nnjity stomach, soars aloft and sweejjs the air in quest of food, the muscular 

 energies are then strained to the utmost, the heart beats with the most forcibli; and 

 rapid contractions to propel the current of blood along the systemic arteries, and th(; 

 ])ulmonary vessels require the greatest possible supply of hlood to serve the heart 

 with the due quantity of arterialised fluid : the digestive system, on the other liand, 

 is in a state of re]>ose and we may conceive the portal circulation to be at its lowest 

 ebb. Suppose the eagle to be glutted with his quarry and reduced to a state of 

 torpor ; the animal functions are now at rest, but the organic powers concerned in 

 the assimilation of the food are in full play, and the portal or hepatic circulation is 

 as active as was the pulmonary a short time before." But since the same disposition 

 of veins occurs in Ajderijx, Owen admits that " the modifications of this part of tin? 

 venous system were less important than I had lieen led to anticipate in a bird whose 

 comparatively limited powers of locomotion must he attended with less jjartial and 

 excessive action of the respiratory system than in birds of flight" ("On the Anatomy 

 of the Southern Apteryx," Trans. Zool. Soe. vol. ii. 1841). Moreover, the theory 

 obviously does not apph' to the similar venous arrangement found in Cerato'lus, 

 7\mphibia, and Re])tilia. In mammals, on the other hand, which most resemble 

 birds in the alternating activity of the respiratory and jjortal systems, a communi- 

 cation between the ])ortal and post-renal \eins does not exist ! 



