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, w) 
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- 
Fia, 22,—Diagrammatic plan of the inferior surface of a multilobed liver of a Mammal. 
The posterior or attached border is uppermost. 2, Umbilical vein of the foetus, represented by 
the round ligament in the adult, lying in the umbilical fissure ; dv, the ductus venosus ; ve, 
the inferior vena cava ; p, the vena porte entering the transverse fissure ; Uf, the left lateral 
fissure ; rlf, the right lateral fissure ; cf, the cystic fissure ; 11, the left lateral lobe ; Ic, the left 
central lobe ; re, the right central lobe ; 71, the right lateral lobe ; s, 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, rif and Jif). When these are more 
deeply cut than the umbilical fissure (uv), 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 (rc and 
le) separated by the umbilical fissure, and those beyond the lateral 
fissures on each side being the right and left lateral lobes (71 and J/). 
