THE DIGESTIVE SYSTEM 



61 



describing them without some more general system. In order to 

 arrive at such a system it appears desirable to consider the liver in 

 all cases as primarily divided by the umbilical vein (see Fig. 22, u) 

 into two segments, right and left. This corresponds with its 

 development and with the condition characteristic of the organ in 

 the inferior classes of vertebrates. The situation of this division 

 can almost always be recognised in adult animals by the persistence 

 of some traces of the umbilical vein in the form of the round 

 ligament, and by the position of the suspensory ligament. 



When the two main parts into which the liver is thus divided 

 are entire, as in Man, the Ruminants, and Cetacea, they may be 

 spoken of as the right and left lobes ; when fissured, as the right 

 and left segments of the liver, reserving the term lobe for the sub- 





Fio. 22. Diagrammatic plan of the inferior surface of a multilobed liver of a Mammal. 

 The posterior or attached border is uppermost, u, Umbilical vein of the foetus, represented by 

 the round ligament in the adult, lying in the umbilical fissure ; dv, the ductus venosus ; we, 

 the inferior vena cava ; p, the vena portae entering the transverse fissure ; llf, the left lateral 

 fissure ; rlf, the right lateral fissure ; c/, the cystic fissure ; U, the left lateral lobe ; Ic, the left 

 central lobe ; re, the right central lobe ; rl, the right lateral lobe ; , the Spigelian lobe ; c, the 

 caudate lobe ; g, the gall-bladder. 



divisions. This will involve no ambiguity, for the terms right and 

 left lobe will no longer be used for divisions of the more complex 

 form of liver. In the large majority of mammals each segment is 

 further divided by a fissure, more or less deep, extending from 

 the free towards the attached border, which are called right and 

 left lateral fissures (Fig. 22, rlf and llf). When these are more 

 deeply cut than the umbilical fissure (u), the organ has that 

 tripartite or trefoil-like form just spoken of, but it is easily seen 

 that it is really divided into four regions or lobes, those included 

 between the lateral fissures being the right and left central (re and 

 Ic) separated by the umbilical fissure, and those beyond the lateral 

 fissures on each side being the right and left lateral lobes (rl and II). 



