1190 



DIGESTIVE SYSTEM 



into the transverse septum in the region where this is attached to the ventral mesoderm of the 

 developing intestine; and that, with further development, the transverse septum differentiates 

 into an upper thinner portion, inclosing the Cuvierian ducts, and destined to form the diaphragm, 

 and a lower thicker portion in which the hver develops. Shortly after the formation of the 

 entodermal bud which forms the liver this mass of epithelium becomes penetrated by out- 

 growths from the omphalo-mesenteric veins, reducing the epithelial mass to anastomosing 

 trabeculse separated by blood-spaces forming a sinusoidal circulation. The definite hepatic 

 lobules are not differentiated until after birth. The process of the development of the lobules 

 is very complicated, the vascular arrangement being shifted repeatedly (Mall). 



The hver rapidly enlarges, filling the upper portion of the abdominal cavity, and extending 

 along its ventral wall to the region of the umbilicus. During the enlargement it in a measure 

 outgrows the transverse septum, and there are developed grooves which result in an infolding 

 of ,the peritoneum covering the transverse septum, and which in part separate the developing 

 liver from that part of the septum destined to form the diaphragm, and also from the ventral 

 abdominal wall. These grooves appear at the sides and also ventral to the liver, but do not 

 completely separate the liver from the diaphragm, nor do they meet in the median line. A 

 portion of the liver, therefore, remains uncovered by peritoneum, and remains attached to the 

 diaphragm; this area may be known as the uncovered or phrenic area of the liver. Around 

 this area the peritoneum of the hver is reflected on to the diaphragm, forming the coronary 

 ligament, with right and left extensions, designated as the right and left triangular hgaments. 

 Owing to the fact that the grooves which develop on the sides of the liver do not meet in the 

 median line, there persists a fold of peritoneum which attaches the liver to the ventral abdominal 



Fig. 952. — Diagram (A): A Sagittal Section of an Embryo showing the Liver en- 

 closed WITHIN THE Septum Transversum; (B) A Frontal Section of the same; (C) Fron- 

 tal Section of a Later Stage when the Liver has separated from the Diaphragm. 



All, Allantois; CI, cloaca; D, diaphragm; Li, liver; Ls, falciform ligament of the liver, M, 

 mesentery; Mg, mesogastrium; Pc, pericardium; S, stomach; ST, septum transversum; U, 

 umbihcus. (McMurrich.) 



wall; this forms the falciform ligament, which divides the superior surface of the liver into a 

 right and a left lobe. The region of the attachment of the ventral mesentery (mesogastrium) 

 into which grows the entodermal liver bud, forms the lesser omentum. The developing liver 

 early comes into intimate relation with the omphalo-mesenteric veins, and a little later the um- 

 bilical veins. The developmental history of these veins and their relation to the developing 

 liver is discussed elsewhere (see Development of the Portal Vein and Inferior Vena 

 Cava, p. 694). After birth the left umbilical vein forms the hepatic ligamentum teres, situated 

 in the free edge of the falciform ligament. The ductus venosus likewise atrophies to form the 

 ligamentum venosum. 



The gall-bladder has its origin in a groove lined by entoderm, which appears on the ventral 

 surface of the primitive intestine or archentcron, between the stomach and the yolk-vesicle. 

 From the cephahc end of this groove grows out the bud destined to form the liver; the caudal 

 end of the groove becomes gradually separated from the developing intestine to form a pouch, 

 lined by entoderm, which forms the beginning of the gall-bladder. With further growth the 

 attachment to the intestine of both the liver and the gall-bladder becomes narrowed to form the 

 ductus choledootius. 



During d(;v(!lopment, the liver undergoes marked changes in form and relative size. It 

 grows with great rapidity in tlie ciiihryo, its maximum relative size reaching 7 to 10 per cent, of 

 the entire body about tiie tliird jjrciiatal month. At this time, the liver is globular in form, the 

 visceral surface very small, and t he l(>ft lobe more nearly approaching the right in size. During 

 the later fo-tal months (fig. 9513) and at birth, the liver forms about 5 per cent, of the whole 

 body. It still remains relatively large in infancy, but decreases to about 2.5 per cent, in the 

 adult. From the beginning, the relative weight of the liver averages slightly higher in the 

 female. 



Variations of the liver and bile passages. — Many variations of the liver have already 

 been mentioned. In .size, both relative and absolute, it is subject to marked individual varia- 

 tions, as well as according to age and sex (i)rcvioiisly described). In /orm, the liver is also quite 

 variable. There are two extreme types: (1) in which the liver is very wide, extending far over 

 into the left hypochondrium, but relatively flattened from above downward; and (2) in which it 



