The primitive form of the liver is symmetrically bilobed, a type which

is seen well in the chelonian organ (Fig. 238).

[Illustration: FIG. 238.--_Pseudemys elegans_, pond turtle. Alimentary

canal. (Columbia University Museum, No. 1437.)]

In size the liver is subject to great variations. It is usually larger

in animals whose food contains much fat. Hence carnivora in general have

a larger liver than herbivorous animals.

Its shape also varies considerably, depending on the form of the body

cavity and on the amount and disposition of the available space. Hence

in the snakes the organ appears long drawn out, flattened, almost

ribbon-like (Fig. 239), while the relatively very large coronal diameter

of the body cavity in the turtles permits the liver to expand

transversely (Fig. 238).

[Illustration: FIG. 239.--Stomach, mid-gut, pancreas, and liver of _Boa

constrictor_, boa. (Columbia University Museum, No. 1832.)]

[Illustration: FIG. 240.--Liver of _Macacus cynomolgus_, Kra monkey.

(Columbia University Museum, No. 28/1833.)]

[Illustration: FIG. 241.--Liver of _Pleuronectes maculatus_, flounder.

(Columbia University Museum, No. 1679.)]

In general, when the liver is large and the available space for its

reception limited, it is usually split into several (two to seven)

lobes, which permit, by mutual displacement, the accommodation of the

organ to varying space-conditions of the body cavity (Fig. 240). Under

the opposite circumstances, on the other hand, even the primitive

bilobed character may disappear and the liver is then unlobed (Fig.

241).

The presence or absence of a gall-bladder depends apparently largely on

the character of the food and on the habitual type of digestion. In many

vertebrates digestion is carried on nearly continuously, without marked

interruption, especially in many ungulates, ruminants and rodents. In

such animals the gall-bladder is absent. It is also absent in several

birds (most Parrots, Doves, Ostrich, Rhea americana, the Cuculidae,

Rhamphastos, etc.). This variability emphasizes the morphological fact

that the biliary bladder is only a modified portion of the hepatic duct

system, as shown by the development above outlined.

A great variety is observed in the arrangement of the biliary ducts,

through which, at the period of intestinal digestion, bile passes from

the liver and gall-bladder into the intestine, while in the intervals of

digestion the secretion is only carried from the liver to the bladder.

The following main types of the biliary duct system may be recognized:

1. The hepatic duct joins the cystic to form the common bile-duct,

entering the duodenum by passing obliquely through the intestinal wall

(Fig. 242). This form is encountered in man and in most mammals. It is

also found in some birds (_Buceros_), many amphibians, and in some fish

(_Lophius_). Instead of one hepatic duct two may join the cystic duct

separately to form the common bile duct (_Phoca litorea_), or the number

of hepatic ducts may be further increased. The separate hepatic ducts

then unite successively with the cystic duct. This occurs in many

mammals (as _Tarsius_, _Galeopithecus_, monotremes) and in some fishes

(_Xiphias_, _Trigla_, _Accipenser_) (Fig. 243).

[Illustration: FIG. 242.--Schema of hepatic and cystic ducts. (Nuhn.)]

[Illustration: FIG. 243.--Schema of hepatic and cystic ducts. (Nuhn.)]

2. Of two hepatic ducts only one helps to form with the cystic duct the

common duct, while the other leads from the liver transversely into the

bladder, especially into the neck, forming the hepatico-cystic duct

(Fig. 244). This arrangement is found in several mammals (calf, sheep,

dog).

[Illustration: FIG. 244.--Schema of hepatic and cystic ducts. (Nuhn.)]

[Illustration: FIG. 245.--Schema of hepatic and cystic ducts. (Nuhn.)]

3. No common bile-duct is formed. The hepatic and cystic ducts each

empty separately into the intestine (hepato-enteric and cysto-enteric

ducts), while a hepato-cystic duct carries the bile directly from the

liver to the gall-bladder (Fig. 245).

_Lutra vulgaris_ among mammalia, the majority of the birds and several

reptilia present this type.

When the gall-bladder is absent a single large hepato-enteric duct is

found, or instead a number of smaller ducts which enter the intestine

successively.

=I. C. Development of Vascular System of Liver.=--In order to comprehend

the peritoneal relations of the adult liver it is absolutely necessary

to have a clear understanding of the development of the vascular system

in connection with the gland.

For our purpose, in the first place, a serial consideration of the

successive stages, illustrated by schematic diagrams, will prove most

practicable. These diagrams represent the structures in the dorsal view,

_i. e._, in the position which they would occupy in the adult liver with

the gland resting on its upper or convex surface and with the ventral

sharp margin turned toward the beholder (see Fig. 259).

The development of the venous system, especially in connection with the

liver, presents a somewhat complicated series of successive conditions.

After having become familiar with the principal typical embryonal

stages, as shown in the following diagrams, the student is strongly

recommended to cement this knowledge by the comparative examination of

the venous system. The permanent veins of the lower vertebrates, while

in many cases not strictly homologous to those of the higher forms, yet

are excellent objects for study, since they serve to illustrate

temporary stages in the development of the mammalian venous system, and

to that extent are of aid in comprehending one of the most difficult and

important chapters in human anatomy. At the conclusion of the

diagrammatic consideration of the mammalian development a number of

comparative facts will be put together for this purpose.

=1. Early Stage.=--In the earlier developmental stages in mammalian

embryos the primitive dorsal aorta extends caudad along the ventral

aspect of the vertebral axis, giving off paired vitelline or

omphalo-mesenteric arteries to the yolk-sac and allantoic arteries to

the embryonic urinary bladder or allantois (Figs. 246 and 247).

[Illustration: FIG. 246.--Diagram of embryonic vascular system, without

the portal circulation. (Parker, after Wiedersheim.) The dorsal aorta is

formed by the junction of the right and left aortic roots arising from

the confluence of the branchial arterial arches.]

The blood is returned from the vascular area of the yolk-sac by two

vitelline or omphalo-mesenteric veins, which unite near the heart to

form a common trunk, continued as the _sinus venosus_ into the caudal or

auricular extremity (venous end) of the primitive tubular heart (Figs.

246, 247 and 248).

[Illustration: FIG. 247.--Diagram of the circulation of the yolk-sac at

the end of the third day of incubation in the chick. (After Balfour.)

The median portion of the first aortic arch has disappeared; but its

proximal end forms the external, its distal the internal carotid

arteries. The whole blastoderm has been removed from the egg and is

viewed from below. Hence the left appears on the right, and _vice

versa_.

Arteries in black.

Veins in outline.]

[Illustration: FIG. 248.--Schema of vitelline veins.]

=2. Development of Allantois. Stage of Placental Circulation.=--The

placental circulation, replacing the temporary vitelline circulation of

the earliest stages, is inaugurated by the appearance of two umbilical

veins, which pass cephalad, imbedded in the tissue of the ventral

mesogastrium, to empty into the sinus venosus near the vitelline veins

(Fig. 249). The umbilical veins return the oxygenated blood from the

placenta to the embryo. At first the right umbilical vein is the larger

of the two.

[Illustration: FIG. 249.--Schema of umbilical veins, early stage.]

The sinus venosus at this time also receives two large veins,

transversely directed, called the ducts of Cuvier, which are formed near

the heart by the union of the anterior cardinal (primitive jugular) and

posterior cardinal veins, draining respectively the head end of the

embryo, and the body walls and Wolffian bodies.

The vitelline veins are placed on each side of the primitive small

intestine, and become connected with each other by a broad anastomotic

branch (Fig. 249). When the hepatic outgrowth buds from the duodenum the

vitelline veins send out branches which break up into a wide-meshed

capillary network in the mesodermic tissue enveloping the hepatic

cylinders. Hence at this period the circulation in the vitelline veins

is made up of three districts:

(_a_) Distal segment of veins, coursing along duodenum, and joined by a

transverse anastomosis, before reaching the liver bud (subintestinal

veins).

(_b_) Middle segment, from which capillary vessels are derived,

ramifying upon and between the developing hepatic cylinders.

(_c_) Proximal segment, formed by the continuation of the proximal part

of the vitelline veins into the sinus venosus of the heart.

[Illustration: FIG. 250.--Schema of primitive portal circulation.]

=3. Formation of Portal Circulation. A.=--With the further development

of the liver the direct connection of the distal segment of the

vitelline veins with the sinus venosus becomes lost, the intermediate

segment being entirely broken up into an intrahepatic network (Fig.

250). Hence all the blood brought to the liver by the vitelline veins

(venae hepaticae advehentes) passes through the hepatic capillary

circulation, before it is carried by the proximal segment of the

vitelline veins (venae hepaticae revehentes) into the sinus venosus. The

amount of this blood increases with new connections which the vitelline

veins make with the venous radicles developing in the intestinal tract

and its appendages. In proportion as, with the development of the

placenta and reduction of the yolk-sac, the original significance of the

vitelline veins as nutritive and respiratory vessels disappears, this

secondary connection of the vitelline veins with the veins of the

alimentary tract becomes more and more important, until finally the

original vitelline veins, now properly called omphalo-mesenteric veins,

return the blood from the intestinal tube, pancreas and spleen to the

liver.

The venae hepaticae advehentes, becoming connected in this way with the

developing intestine, pancreas and spleen, form the rudiments of the

future portal system, while the venae hepaticae revehentes are prototypes

of the hepatic veins of the adult circulation.

=B. Development of the Portal Vein.=--The distal subintestinal segments

of the vitelline veins are early united by a transverse anastomotic

branch. The section of the veins above this anastomosis is seen already

in Fig. 250 to have assumed an annular shape, while the veins below the

primary anastomosis are approaching each other to form a second

ring-like junction.

In Fig. 251 the subintestinal segments of the two vitelline veins are

seen to have communicated with each other by transverse anastomotic

branches around the duodenum, two of these branches being situated

ventrad and one dorsad of the intestinal tube. These branches, and the

portions of the primitive vitelline veins between their points of

derivation, form two vascular loops or rings, encircling the primitive

duodenum (Fig. 251).

[Illustration: FIG. 251.--Schema of further development of portal

circulation and connection of same with umbilical veins in early

stages.]

The distal portions of the vitelline veins, before reaching the caudal

annular duodenal anastomosis, next fuse into a single longitudinal

vessel which also receives the veins from the stomach, intestine,

spleen, and pancreas, and forms the beginning of the portal vein.

By atrophy of the right half of the lower, and of the left half of the

upper duodenal venous ring (Figs. 252 and 253), the proximal portion of

the portal vein is formed as a single vessel, taking a spiral course

around the duodenum (Fig. 256). Hence in the adult the portal vein and

its principal branch (the superior mesenteric vein) crosses over the

ventral surface of the duodenum (third portion), turns along the mesal

side of the second portion, and then continues to the liver along the

dorsal aspect of the first portion (Fig. 254). _Note_--In comparing Fig.

254 with the schematic figures it should be noted that the same presents

the parts in the _ventral_ view, while the schemata offer the _dorsal_

aspect.

[Illustration: FIG. 252.--Second stage in development of circulation

through portal and umbilical veins. The proximal segment of the main

portal vein is formed by the persistence of the left half of the distal

and right half of the proximal periduodenal vascular ring of the

omphalo-mesenteric veins. The distal segment of the main portal vein is

the product of the fusion of the omphalo-mesenteric veins, and becomes

connected with the veins of the intestinal canal, pancreas, and spleen.

The proximal terminal segment of both umbilical veins becomes included

in the system of the venae hepaticae revehentes.]

[Illustration: FIG. 253.--Third stage in development of portal and

umbilical veins during the placental period.]

[Illustration: FIG. 254.--Corrosion preparation showing course of portal

vein and tributaries in relation to duodenum. (Columbia University

Museum, No. 1857.)]

[Illustration: FIG. 255.--Human embryo of 10 mm. cervico-coccygeal

measure. Heart and ventral body-wall removed to show sinus venosus and

entering veins. (Kollmann, after His.)]

[Illustration: FIG. 256.--Final stage of development of portal and

umbilical veins in the placental period.]

=4. Changes Leading to the Final Arrangement of the Umbilical Veins.=--A

very important rearrangement of the umbilical veins takes place. These

veins originally course in the lateral abdominal wall, close to the fold

of the amnion (Fig. 255), and then turn cephalad of the developing liver

along the septum transversum to empty into the sinus venosus at each end

(Figs. 249 and 250). The right umbilical vein is at first the larger.

This symmetrical arrangement, and the direct connection of the umbilical

veins with the sinus venosus, now becomes lost by the occurrence of the

following changes:

1. At first (Fig. 249) all the blood carried to the liver by the

omphalo-mesenteric veins passes through the hepatic capillary network

before being conducted by the venae revehentes to the sinus venosus. Very

early, however, a new intrahepatic channel develops, the ductus venosus

(Figs. 250-253), which passes obliquely between the entrance of the left

omphalo-mesenteric vein into the capillary system (l. v. advehens) and

the termination of the right omphalo-mesenteric vein (r. vena revehens)

in the sinus venosus.

In human embryos of 4 mm. the ductus venosus can already be

distinguished, and in embryos of 5 mm. the vessel has assumed

considerable proportions.

2. A communication is next established on both sides between the

capillary hepatic network in the portion of the liver nearest to the

abdominal wall and the umbilical veins as they ascend imbedded in the

abdominal wall (Fig. 251).

This connection is usually from the start larger on the left side and

connects with the left omphalo-mesenteric vein just at the point where

the same is about to be continued into the ductus venosus. This

connection becomes rapidly larger, so that the ductus venosus, which at

first appeared merely as an anastomotic channel between the left

omphalo-mesenteric vein and the terminal portion of the right

omphalo-mesenteric vein, now forms the main continuation of the left

umbilical vein. This vessel grows very rapidly up to its connection with

the ductus venosus and soon exceeds the right umbilical vein in size

(Fig. 252). Beyond the ductus venosus on the other hand the proximal

segment of the left umbilical vein diminishes in size, and loses its

independent character by incorporation in the hepatic circulation. Only

its terminal portion, emptying into the sinus venosus, is preserved.

This is surrounded by the growing masses of hepatic cylinders and is

converted into a vena revehens.

The connection of the right umbilical vein with the liver vessels is at

first symmetrical to that on the left side, but less strongly developed.

The effect of this connection is to reduce in the same way the proximal

segment of the right umbilical vein and to convert its termination into

a vena revehens. With the great development of the left vein, however,

the vein on the right side gradually diminishes and finally loses its

connection with the intrahepatic circulation altogether. The right

umbilical vein is now reduced to a vessel of the ventral abdominal wall,

which carries blood in the reverse of the original direction, _i. e._,

from the abdominal wall caudad _into_ the left umbilical vein (Figs. 253

and 255).

The connection thus established between the umbilical vein and the

portal circulation results in the formation of a single large (the

original left) umbilical vein which, throughout the remainder of foetal

life, returns all of the placental blood (Fig. 253).

The newly developed hepatic portion of the left umbilical vein becomes,

however, not only connected with the ductus venosus, but also with the

right part of the upper venous ring, derived from the right

omphalo-mesenteric vein (Fig. 253). This connection forms the left

portal vein of the adult, and enlarges rapidly.

The terminations of the ductus venosus and of the venae hepaticae

revehentes undergo a number of secondary changes in relative position.

The left hepatic vein loses its direct connection with the sinus

venosus, and now opens into the termination of the ductus venosus, into

which the right hepatic vein also empties. This common vessel (v.

hepatica communis) subsequently forms the proximal segment of the

postcava when this vessel develops (Fig. 256).

The blood, therefore, returned to the liver by the left umbilical vein

divides at the transverse fissure into three streams. Two of these pass

through the connection with the portal vein and through branches

developed from the hepatic part of the umbilical vein into the capillary

system of the right and left lobe. The third continues through the

ductus venosus to the common hepatic vein and sinus venosus (Fig. 256).

The ductus venosus thus becomes the chief vessel returning arterialized

placental blood to the heart. When the postcava develops fully the

hepatic segment of this vessel also joins the terminal part of the

ductus venosus (Fig. 256) and gradually replaces the same as the main

returning venous channel, the proximal part of the ductus venosus being

incorporated in the vena cava (Fig. 257). The postcava then receives the

right hepatic veins separately, while the left hepatic veins and ductus

venosus open together into the main vein. This condition obtains up to

the time of birth and the consequent interruption of the placental

circulation.

[Illustration: FIG. 257.--Schema of relation of postcava to hepatic

veins and ductus venosus.]

While at first the ductus venosus communicates throughout its entire

length with the meshwork of the hepatic capillary system, a separation

into two segments, _i. e._, ductus venosus proper and intrahepatic

segment of umbilical vein, is established after the free communication

with the left umbilical vein takes place. This condition is exhibited in

Fig. 258, which represents the corroded venous system of the foetal

liver, and in Fig. 259, showing an injected liver in the foetus at term.

[Illustration: FIG. 258.--Corrosion preparation of venous system of

human liver in foetus at term. (Columbia University Museum, No. 1834.)]

[Illustration: FIG. 259.--Injected and hardened human liver from foetus

at term. (Columbia University Museum, No. 1853.)]

It will be observed that the umbilical vein on entering the liver gives

off a large branch to the left lobe, and a smaller branch on the right

side to the quadrate lobe, which act as the main venae advehentes of

these portions of the liver. Arrived at the transverse fissure the

umbilical vein divides into three branches, at right angles to each

other. The left branch enters the left lobe, the right branch becomes

directly continuous with the left main division of the portal vein,

while the central branch, continuing the direction of the umbilical

vein, passes dorsad, as the ductus venosus proper, to join the left

hepatic vein close to its entrance into the postcava.

=5. Changes Consequent upon the Establishment of Pulmonary

Respiration.=--After birth the umbilical vein and its continuation, the

ductus venosus, become obliterated, the former constituting the round

ligament of the liver, the latter the ligament of the ductus venosus,

both structures imbedded in corresponding portions of the sagittal

fissure on the caudal and dorsal surfaces of the adult liver (Figs. 284

and 286). The lateral branches of the umbilical vein, however, in its

course from the ventral margin of the liver to the transverse fissure

(Fig. 258), remain pervious and are transferred to the portal

circulation.

[Illustration: FIG. 260.--Diagram of intrahepatic foetal venous

circulation.]

[Illustration: FIG. 261.--Diagram illustrating the changes in the

intrahepatic venous circulation resulting from the cessation of the

placental circulation at birth.]

It will be noticed, in reference to the _direction_ of the blood

current, that at birth a sudden reversal takes place in the right

terminal branch of the umbilical vein at the transverse fissure (Figs.

260 and 261). Before birth the blood current of the umbilical vein

divides into three streams, right, left and central. The latter enters

the ductus venosus. The left enters the liver directly, the right

traverses, from left to right, the segment between the termination of

the umbilical and the bifurcation of the portal vein. This segment in

the adult carries blood from right to left, as left branch of the portal

vein. In the foetus, however, the blood traverses this segment from left

to right, in passing from the umbilical to the right branch of the

portal vein. The blood entering the liver through the portal vein passes

chiefly into the right division of that vessel (Fig. 260).

After birth all the venous blood entering the liver passes

through the portal vein. In the right division the direction of the

current is the same as in the foetus.

On the left side, however, the current is now from right to left, from

the bifurcation of the portal into the channels of the left lobe

formerly connected with the umbilical vein (Fig. 261).

Hence the direction of the current in this segment is reversed at birth.

SUMMARY OF HEPATIC CIRCULATION.

The foregoing consideration of the development shows us that the hepatic

circulation presents successively three main stages:

=1. Omphalo-mesenteric or Vitelline Stage=, which results in the laying

down of the primary capillary circulation of the liver and in the

establishment of its connection with the developing veins of the

alimentary tract (primitive portal channels).

=2. Umbilical or Placental Stage=, in which the greater part of the

blood circulating through the liver is oxygenated blood returned from

the placenta by the umbilical vein, accounting for the rapid growth and

relatively large size of the organ during foetal life.

The placental blood uses the preformed capillary channels of the

vitelline or primitive portal system in the liver, and the same rapidly

extend and enlarge with the accelerated growth of the gland. During this

stage venous blood is also returned from the alimentary tract to the

liver by the portal vein, produced by fusion of the distal segments of

the primitive vitelline veins and their secondary connection with the

mesenteric, splenic and pancreatic veins (omphalo-mesenteric development

of primitive vitelline veins).

=3. Adult or Portal Stage.=--With the interruption of the placental

circulation the portal vein assumes again its original position as the

only vein carrying blood to the liver. With the establishment of

intestinal digestion and absorption this vessel grows rapidly in size.

COMPARATIVE ANATOMY OF THE HEPATIC VENOUS CIRCULATION.

For the purpose of fixing the main facts in connection with the

development of the higher mammalian hepatic circulation, and in order to

obtain a demonstration of the cycle through which the different veins

pass, the student is recommended to examine, preferably by personal

dissection, a limited series of lower vertebrates which can be readily

procured and easily injected. The following series has been selected,

but it will be understood that other forms can be substituted, according

to the local conditions which govern the supply of the material.

1. _Fish._ _A Selachian_, the common skate (_Raja

ocellata_) or dog-fish (_Acanthias vulgaris_).

2. _Amphibian._

(_a_) Urodele. _Necturus maculatus._

(_b_) Anura. The common _frog_.

3. _Reptile._

Preferably, on account of the ease of injection, one of the larger

lizards, as _Iguana tuberculata_.

The turtles, although somewhat more difficult objects to prepare, can be

substituted.

4. _Bird._ The common fowl.

5. Human foetus at term.

=1. Fish.=--The venous system can be injected by tying a canula in the

lateral vein, and injecting both cephalad and caudad, or by injecting

cephalad through the caudal vein. The injection of the systemic veins

can also be made caudad through one of the ducts of Cuvier, combined

with an injection cephalad of the caudal vein.

[Illustration: FIG. 262.--Diagram of the veins of a selachian.

(Wiedersheim, after Parker.)

The lateral vein arises from a venous network surrounding the cloaca,

receiving one or more cutaneous veins of the tail, veins of the

body-wall, and veins of the pelvic fins.

The caudal vein divides at the posterior end of the kidney into the two

renal-portal veins, from which the advehent veins of the renal-portal

system are derived. The revehent renal-portal veins join to form the

posterior cardinal veins, which, after dilating enormously to form the

cardinal sinuses, join with the anterior jugular, subclavian, and

lateral veins to form the ducts of Cuvier. The latter receive the

inferior jugular veins, from the deep parts of the head and neck and the

terminations of the hepatic portal system (hepatic sinus).

The hepatic portal vein is formed by the veins of the oesophagus,

stomach, and intestines. After traversing the capillary vessels of the

liver, the revehent hepatic veins unite to form an extensive hepatic

sinus before entering the heart.]

The following main facts are to be noted in the venous system of the

Selachian (Fig. 262):

=1. There are Two Portal Systems.= (_a_) _Renal Portal System._--The

caudal vein divides near the vent into two branches which course along

the lateral border of the kidneys, sending _afferent_ or _advehent_

veins into the organ. The blood traverses the renal capillaries and is

gathered together by the _efferent_ or _revehent_ veins, which empty

into median paired vessels, the posterior cardinals.

(_b_) _Hepatic Portal System._--The veins of the digestive tract and

appendages unite to form a hepatic portal vein. The blood after

traversing the capillary system of the liver is collected by hepatic

veins, which form a dilated hepatic sinus emptying into the sinus

venosus of the heart.

2. The middle segment of the intestine, presenting a spiral valve in the

interior, gives rise to a vein emptying into the portal vein which

corresponds to the subintestinal vitelline vein of the mammalian embryo

(Fig. 202).

3. The posterior cardinal veins, also greatly dilated and forming the

posterior cardinal sinus, join, near the heart, the veins returning

blood from the head, the anterior cardinal or jugular, to form a

transversely directed trunk, the duct of Cuvier, which empties into the

sinus venosus at the auricular extremity of the heart. Into the duct of

Cuvier empties on each side a _lateral vein_ returning the blood from

the body walls. This vein can be considered, for our present purpose, as

representing in general the abdominal vein of amphibians and reptiles,

and the umbilical vein of the mammalian embryo.

The adult selachian venous system is therefore to be considered as

illustrating the following conditions above encountered in our study of

the embryology of the mammalian venous system.

1. The heart illustrates excellently the stage in the mammalian

development, in which auricular and ventricular segments have

differentiated, but before the division of the cavities into a pulmonary

and systemic portion by the development of the auricular and ventricular

septa and the division of the arterial trunk into pulmonary artery and

aorta.

The sinus venosus still exists, as an ante-chamber to the auricular

cavity proper, receiving on each side the ducts of Cuvier, which

represent the fusion product of the systemic veins, anterior and

posterior cardinal.

2. The hepatic portal circulation corresponds to the mammalian stage in

which the vitelline veins have become omphalo-mesenteric by joining the

intestinal veins.

The spiral vein remains as a portion of the original vitelline vein

corresponding to the subintestinal segment of the mammalian embryo (cf.

Figs. 248 and 249).

The selachian portal vein represents the united vitelline veins, into

which the veins of the digestive tract open.

In the liver we find a simple system of venae advehentes, derived from

the branching of the portal vein, a hepatic capillary network, and venae

revehentes, the proximal remnants of the original vitelline veins which

carry the liver blood to the sinus venosus. The condition of the hepatic

circulation corresponds therefore to the stage shown in Fig. 250 of the

mammalian development. There is as yet no association of the hepatic

venous system with the representative of the umbilical vein (the lateral

vein of the selachian).

3. The lateral veins, which we can, as stated, regard for purposes of

illustration, without prejudging their genetic significance, as

representing the mammalian embryonic umbilical veins, still present the

condition corresponding to the early mammalian embryonal stage shown in

Fig. 250. They are veins of the body walls, emptying cephalad of the

liver, directly into the ducts of Cuvier, and through them into the

sinus venosus of the heart.

Fig. 262 shows the arrangement of the venous system in a typical

selachian diagrammatically.

=2. Amphibian.= (_a_) =Urodele.=--The following points are to be noted

in comparison with the preceding form:

1. The two ducts of Cuvier entering into the sinus venosus are formed by

the anterior cardinal and subclavian veins, which latter, having

appeared with the full development of an anterior extremity, receives

the posterior cardinal veins, representing the mammalian azygos system.

2. The renal portal circulation persists. The caudal vein is, however,

no longer the only afferent vein of this system. With the full

development of a posterior extremity an iliac vein returns the blood

from the same and gives a large branch (afferent to the portal renal

system), while the trunk continues cephalad as an anterior abdominal

vein, corresponding to the lateral selachian vein, emptying in the

hepatic portal vein.

3. The efferent veins of the renal portal system no longer unite to form

the posterior cardinal, as in the Selachian, but empty into a new median

vessel, the inferior vena cava, or postcava, which has replaced the

distal segments of the posterior cardinal veins.

The postcava now carries the blood from the kidneys directly to the

heart. The original posterior cardinal veins still persist in their

proximal segments, as smaller trunks connecting the distal part of the

postcava with the ducts of Cuvier through the subclavian veins. The

ducts of Cuvier represent the precavae (venae cavae superiores) of mammalia

and the postcardinals the mammalian azygos veins.

4. The hepatic portal system differs in two respects from the Selachian

type.

(_a_) The blood returned to the liver from the digestive tract by the

portal vein becomes mixed before entering the gland with the blood

returned from the posterior extremities and abdominal walls by the

abdominal vein.

This vein, paired below and continuous with the lateral of the two

branches into which the iliac vein divides, becomes united into a single

trunk above and empties into the portal vein.

The abdominal vein represents the lateral vein of the Selachian and

corresponds to the umbilical vein of the higher vertebrates.

(_b_) The venae hepaticae revehentes do not empty directly into the sinus

venosus, but into the proximal portion of the postcava.

Hence the adult urodele venous system illustrates, in reference to the

mammalian development, these stages:

1. The umbilical (abdominal) vein has lost its direct connection with

the sinus venosus. The proximal segment, cephalad of the liver, has

disappeared, and its blood now passes directly into the hepatic

circulation by its union with the portal vein.

(Cf. stage schema Figs. 251 and 252.)

2. The postcaval vein has made its appearance, largely replacing the

posterior cardinal veins, whose proximal segments became converted into

secondary vessels (azygos) uniting the system of the postcava with that

of the duct of Cuvier (mammalian praecava), while their distal segments

are transformed into the distal portion of the postcava.

The postcava, therefore, is made up of two districts:

(_a_) The proximal portion is a new vessel, developed in connection with

the hepatic venous system.

(_b_) The distal portion is derived from the distal segments of the

original posterior cardinal veins.

The termination of the hepatic veins in the postcava corresponds to the

stage shown in schema Fig. 256.

Fig. 263 gives a schematic representation of the arrangement of the

venous system in a typical urodele amphibian (_Salamandra maculosa_).

[Illustration: FIG. 263.--Diagram of the veins of urodele amphibian

(_Salamandra maculosa_). (Wiedersheim.)

The caudal vein bifurcates at the posterior extremity of the kidneys to

form the afferent trunks of the renal-portal system along the lateral

border of the kidneys, from which the advehent veins of the renal-portal

system are derived. The iliac or femoral vein divides into an anterior

and a posterior branch, the latter opening into the afferent

renal-portal vein, while the former, uniting with the one of the

opposite side, forms the abdominal vein, and receives vessels from the

bladder, cloaca, and end-gut. The revehent veins of the renal-portal

system, emerging upon the ventral surface of the kidneys, empty into a

single median vessel, the distal or renal section of the postcava or

vena cava inferior. Proceeding cephalad, the proximal or hepatic section

of this vessel, after traversing the liver and receiving the revehent

hepatic veins of the hepatic portal system, empties into the sinus

venosus of the heart. Previous to entering the liver the postcava gives

off the two posterior cardinal or azygos veins, which continue cephalad,

receiving tributary segmental veins from the body-walls and reach the

sinus venosus by joining the subclavian veins. These latter uniting with

the anterior cardinal (jugular) veins form the ducts of Cuvier (precaval

veins).

The abdominal vein continues cephalad in the ventral mesogastrium to the

liver, giving off a number of smaller branches, which enter the hepatic

portal circulation by penetrating the ventral surface of the liver

between the layers of the ventral mesogastrium, while the main

continuation of the vessel joins the hepatic portal vein at its point of

entrance into the liver.

The hepatic portal vein is formed by tributaries returning the blood

from the digestive tract (intestinal canal, spleen, pancreas). The

blood, after traversing the hepatic portal circulation, is conducted by

the hepatic revehent veins to the proximal section of the postcava. A

number of secondary or accessory portal veins pass from the anterior

portion of the intestinal canal (oesophagus, stomach) directly to the

liver.]

In Fig. 264 the dissected venous system of _Necturus maculatus_, the mud

puppy, is shown in an injected preparation.

[Illustration: FIG. 264.--Dissection of veins of _Necturus maculatus_,

mud-puppy. (Columbia University Museum, No. 1835.)

The postcava has been divided at the cephalic end of the liver just

before entering the sinus venosus, and the postcardinals have been cut

prior to their junction with the subclavian veins.

The stomach has been turned caudad. The abdominal vein has been divided

after the common trunk has been formed by branches from the iliac veins.

The latter are seen entering the afferent renal-portal vein, derived

from the bifurcation of the caudal vein, along the lateral border of the

kidneys.

The junction of the main trunk of the abdominal vein with the hepatic

portal vein takes place close to the liver under cover of the pancreas.

A series of accessory portal veins continuous with the abdominal vein

enter the ventral surface of the liver between the layers of the ventral

mesogastrium. The inter-renal segment of the postcava receives the

revehent renal-portal veins. The iliac vein enters the advehent

renal-portal veins derived from the caudal vein.]

[Illustration: FIG. 265.--Venous system of _Rana esculenta_, frog.

(Ecker.)]

(_b_) =Anure.=--The venous system of _Rana esculenta_ is shown in Fig.

265. Comparison with venous system of _urodele_:

1. The abdominal vein, corresponding to the mammalian umbilical vein,

has assumed a greater importance in reference to the hepatic

circulation. It is a large trunk, continuous below with the pelvic vein,

terminating above in two branches, which enter the liver as afferent

veins, being joined just prior to the division by the hepatic portal

vein.

2. A small cardiac vein, coming from the heart, empties into the angle

of bifurcation of the abdominal vein.

3. The postcava is well developed, formed by large efferent renal veins.

It entirely replaces the posterior cardinal veins which are absent in

the adult animal.

4. A right and left praecaval vein is formed by the union of two jugular

trunks with the vein of the anterior extremity and a large

musculo-cutaneous vein.

Comparison with the mammalian development: the venous system of this

amphibian can be used to illustrate the mammalian embryonal stage shown

in schema Fig. 252, in which the abdominal or umbilical vein has become

the most important vessel in the afferent hepatic venous system.

The communication existing by means of the cardiac vein between the

heart and the hepatic afferent system may suggest, but _purely for

illustrative purposes_, the direct connection of the umbilical vein with

the heart by the ductus venosus in the mammalian embryo (cf. schema

Figs. 250-256).

=3. Reptile.=--In _Iguana_ the renal portal system is well developed.

The caudal vein, returning the blood from the tail and the cavernous

tissue of the genital organs, continues for a short distance upon the

fused caudal end of the two kidneys (Fig. 269) and then divides into two

afferent renal veins which ascend on the ventral surface of the glands,

giving branches to the renal capillary system. About the middle of the

kidney each afferent vein is joined by a large transverse branch from

the abdominal vein (Fig. 266).

[Illustration: FIG. 266.--Systemic veins of _Iguana tuberculata_. The

alimentary canal and appendages, together with the hepatic portal vein

and the intrahepatic segment of the postcava, have been removed. The

liver occupies the space between the divided ends of the postcava. The

vertebral vein represents the rudimentary proximal segment of the

postcardinal vein corresponding to the mammalian azygos vein. (Columbia

University Museum, No. 1320.)]

[Illustration: FIG. 267.--Veins of _Iguana tuberculata_. Connection of

systemic veins with sinus venosus of heart. The rudimentary system of

the vertebral (azygos) veins and their proximal connection with the

subclavian vein are shown. (Columbia University Museum, No. 1859.)]

[Illustration: FIG. 268.--Corrosion preparation of venous system of

liver in _Iguana tuberculata_. The hepatic portal system and its

connection with the abdominal vein, as well as the relation to the

postcava, are shown. The preparation supplements Fig. 266, showing the

parts which have been removed in the latter. (Columbia University

Museum, No. 1860.)]

[Illustration: FIG. 269.--_Iguana tuberculata_, male. Genito-urinary

tract, dorsal view, with renal-portal, postcardinal, and postcaval

veins. (Columbia University Museum, No. 1862.)]

The renal efferent system begins by a number of inter-renal anastomoses

which unite along the mesal border of the right kidney into a large

ascending trunk, while the corresponding vessel of the left side,

starting from the same anastomosis, is considerably smaller (Figs. 266

and 269). Each of these vessels also receives blood from the testis,

epididymis, vas deferens and adrenal body in the male, and from the

ovary and oviduct in the female. They represent, in fact, the distal

functional part of the right and left embryonic postcardinal vein. Just

caudad of the left testis the vein of the left side crosses obliquely

ventrad of the aorta and joins the right vessel to form the trunk of the

postcava, which enters, immediately beyond the cephalic pole of the

right testis, the prolonged caval lobe of the liver (Figs. 266 and 269).

Ascending in the substance of this gland and receiving the afferent

hepatic veins (Fig. 268), the vena cava emerges from the cephalic

surface of the liver greatly enlarged and proceeds to the right

auricle.

The abdominal vein divides below into two branches which pass caudad on

each side of the bladder, receiving tributaries from the same, to the

lateral border of the kidneys (Figs. 266 and 269). Here the vessel is

connected by the transverse branch above described with the afferent

renal portal system derived from the caudal vein. At the same point it

receives the sciatic vein, the principal venous vessel of the posterior

extremity. Above, the main abdominal vein, resulting from the union of

the two branches referred to, ascends on the dorsal surface of the

ventral abdominal wall, receiving a few twigs from the ventral

mesogastrium within whose free caudal edge the vessel runs. Just before

reaching the liver the abdominal vein turns dorsad on the caudal surface

of the gland and joins the hepatic portal vein (Figs. 268 and 275).

Several accessory veins, two or three in number, belonging to the system

of the abdominal vein, pass above this point from the ventral body wall

between the layers of the ventral mesogastrium, to enter the liver

separately on its convex ventral surface, above the fusion of the main

abdominal vein with the portal vein. These additional branches on

entering the liver join the portal system, forming a set of ventral

accessory portal veins.

The hepatic portal vein derives its principal tributaries from the

splenic, gastric, pancreatic and intestinal veins. One or two additional

branches (accessory vertebral portal veins), as above stated, connect

the system of the segmental and vertebral veins with the portal

circulation, entering the liver separately. In like manner one or two

gastric veins (accessory gastric portal veins) enter the dorsal aspect

of the liver separately, passing from the stomach to the gland between

the layers of the gastro-hepatic omentum (Fig. 275).

Compared with the development of the mammalian type, the venous system

of Iguana serves to illustrate the stage in the history of the umbilical

vein (represented by the abdominal vein of the reptile) in which the

connection of the vessel with the portal vein has been formed and

transmits the greater part of the blood returned by the

umbilical vein to the liver, while the proximal segment above this

point, originally continued into the sinus venosus, has begun to

disappear, being, however, still represented by the vessels which, as

accessory ventral portal veins, pass in the ventral mesogastrium, from

the body wall to the liver.

It will be noted that all the hepatic portal blood, whether conducted by

the main portal and abdominal vein, or by the accessory portal branches,

traverses the capillary circulation of the liver before entering the

postcava.

The vertebral and segmental venous system, representing the azygos veins

of the mammalia, is very rudimentary (Figs. 266 and 267). The distal

portions of the postcardinal veins form the efferent renal branches and

the ascending trunks of the postcava.

The next segment of the vertebral veins appears as a trunk on the right

side which enters the portal circulation. A second vein higher up is

connected with both the gastric portal system and with the longitudinal

chain of the vertebral veins. Finally a proximal venous branch on each

side of the vertebral column, representing the upper portion of the

postcardinal veins, receives the proximal segmental veins and empties

into the subclavian vein (Fig. 267).

[Illustration: FIG. 270.--Veins of pigeon, _Columba livia_. (Modified

from Parker and Haswell.) The renal-portal vein of the right side is

supposed to be dissected to show its passage through the right kidney.]

=4. Bird.=--The characteristic change in the venous system of the bird,

as compared with that of the amphibian and reptile, is found in the

nearly complete abolition of the renal portal system. The caudal vein

bifurcates, sending on each side a large trunk, which receives the

pelvic (int. iliac) veins, to the kidney (renal afferent portal vein),

but only a few small branches enter the substance of the gland (Fig.

270, afferent renal V). The main vessel continues cephalad through the

kidney and, after receiving the vein from the posterior extremity

(femoral), unites as common iliac vein with the vessel of the opposite

side to form the postcava. This vessel traverses the liver, receiving

the hepatic afferent veins of the portal system. The portal vein is

formed by tributaries from the intestinal canal, pancreas and spleen,

and is also joined by a large coccygeo-mesenteric vein, which is given

off at the point of bifurcation of the caudal vein and receives

tributaries from the lower part of the alimentary canal. The abdominal

vein of amphibians and reptiles is represented probably by the

epigastric vein, which returns the blood from the omental mass of fat to

the hepatic veins.

Compared with the mammal on the one hand, and with the lower types on

the other, the venous circulation of the bird illustrates the following

points:

1. Extensive reduction of the renal portal system and direct formation

of postcava by the iliac veins, foreshadowing the condition found in the

mammal.

2. Complete separation of the portal and systemic venous circulation in

the adult. Disappearance of the ventral abdominal vein as a vessel of

the body wall.

=5. Human Foetus at Term.=--The student is recommended to examine, by

dissection and injection, the venous system of a foetus at term, noting

the following facts:

1. Course of umbilical vein in ventral abdominal wall and along free

edge of falciform ligament to liver (Fig. 241), corresponding to the

position of the amphibian and reptilian abdominal vein (Figs. 264 and

275).

2. Connection of umbilical vein in liver:

(_a_) With portal system (Figs. 258 and 271).

({~GREEK SMALL LETTER ALPHA~}) With portal vein.

({~GREEK SMALL LETTER BETA~}) With portal system of left and quadrate lobes by

branches derived directly from umbilical vein while

situated in the umbilical fissure (Fig. 258).

(_b_) With hepatic veins and postcava by the ductus venosus

(Figs. 258 and 271).

[Illustration: FIG. 271.--Human foetus at term. Corrosion preparation of

heart and vascular system. (Columbia University Museum, No. 1858.)]

3. Connection of the postcaval and precaval systems by the azygos veins

representing the proximal segments of the embryonic postcardinal veins

(Fig. 272).

[Illustration: FIG. 272.--Human foetus at term. Postcava and azygos

veins. (Columbia University Museum, No. 1861.)]

If possible the dissection of an injected foetus should be combined with

the examination of corrosion preparation of the foetal circulation and

especially of the venous system of the foetal liver (Figs. 258 and 271).

3. The remnants of foetal structures in the adult liver (round ligament

and ligament of the ductus venosus) should be compared with the

structures from which they are derived in the foetus at term (umbilical

vein and ductus venosus).

II. THE VENTRAL MESOGASTRIUM.

This membrane has been heretofore mentioned on several occasions. It now

remains for us to carefully consider its arrangement in detail, both as

regards the peritoneal relations of the liver and in reference to its

influence on the abdominal space as a whole. We can best accomplish this

purpose by considering the membrane in the first place in a purely

schematic manner. In contradistinction to the primitive common dorsal

mesentery, which extends the entire length of the alimentary tube, the

ventral mesentery, or properly the ventral mesogastrium, is confined to

the stomach and proximal portion of the duodenum. We can represent the

membrane as extending between the ventral abdominal wall and the ventral

border (later the lesser curvature) of the stomach and of the hepatic

angle of the duodenum. Cephalad it is connected with the embryonic

septum transversum (future diaphragm). Caudad its two layers pass into

each other in a free concave edge, including between them the umbilical

vein (free edge of falciform ligament of adult). Consequently a

schematic profile or lateral view of the membrane and its attachments in

the earlier stages would appear as represented in Fig. 273, while the

arrangement in transection would be as shown in Fig. 274. It will be

observed that the separation of the cephalic portion of the abdominal

cavity into symmetrical right and left halves, previously indicated in

discussing the primitive stomach and the dorsal mesogastrium, is

actually completed by the ventral mesogastrium. This complete separation

of the lateral halves of the coelom cavity ceases at the point where the

ventral mesogastrium terminates in the free concave edge carrying the

umbilical vein. Hence caudad of this falciform edge the two halves of

the cavity communicate freely with each other ventrad of the intestine

and dorsal mesentery.

[Illustration: FIG. 273.--Schematic profile view of ventral mesogastrium

with developing liver.]

[Illustration: FIG. 274.--Schematic transection of abdomen in region of

ventral mesogastrium.]

This difference in the extent of the mesogastria is perhaps best

understood by reference to their relation to the first portion of the

duodenum. We have seen that the duodenum in the early stages is attached

dorsally by a portion of the common dorsal mesentery, which, after

differentiation of the intestinal tract, immediately follows the dorsal

mesogastrium proper, forming the mesoduodenum (Fig. 172). The proximal

portion of the duodenum (hepatic angle) is still included within the

fold of the ventral mesogastrium which membrane terminates immediately

beyond this point in the free edge surrounding the umbilical vein

(subsequent round ligament) (Fig. 172). The remainder of the duodenum is

devoid of any ventral attachment, being only connected to the dorsal

body wall by the mesoduodenum (Fig. 197).

Subsequently, after the fourth month, while the right surface of the

mesoduodenum and descending duodenum adhere to the parietal peritoneum,

the peritoneal investment of the first portion or hepatic angle remains

free. This peritoneal covering of the proximal duodenal segment is

situated at the point where the caudal end of the ventral mesogastrium,

after surrounding the first portion of the duodenum, becomes continuous

with the dorsal mesentery forming the mesoduodenum. Obliteration of the

latter membrane by adhesion to the parietal peritoneum leaves the first

portion of the duodenum invested on both surfaces by the _lesser

omentum_, derived from the ventral mesogastrium. The ventral surface of

the gut is covered by the ventral layer, the dorsal surface by the

dorsal layer of the lesser omentum. These two layers become continuous

around the right free edge of the lesser omentum (hepato-duodenal

ligament) forming the ventral boundary of the foramen of Winslow (cf.

infra, p. 177).

Returning to the schematic consideration of the ventral mesogastrium

above outlined (Figs. 273 and 274) we have to note the first important

change in the arrangement depending upon the development of the liver.

This organ, growing, as we have seen, from the duodenum, extends between

the two layers of the ventral mesogastrium, receiving a serous

investment from the same. At an early period the liver, developing thus

between the mesogastric layers, reaches the septum transversum and

becomes closely connected with it, laying the foundation for the

subsequent extensive attachment of the gland to the diaphragm.

Extending caudad the liver grows beyond the caudal free edge of the

ventral mesogastrium on each side, carrying the serosa with it.

Consequently the ventral margin of the liver becomes indented at this

point; the umbilical vein and subsequently its fibrous remnant, the

round ligament, are imbedded in a notch and fissure (umbilical notch and

fissure) continued from the ventral margin dorsad along the caudal

surface of the liver (Fig. 259).

This growth of the liver has now effected a division of the primitive

ventral mesogastrium into two segments:

1. Ventral portion, between diaphragm and liver, forms the broad

falciform or suspensory ligament of the liver.

2. The dorsal portion, between liver and stomach, forms the lesser or

gastro-hepatic omentum.

The caudal free edge of the ventral mesogastrium extends between the

umbilicus and the caudal surface of the liver, carrying the umbilical

vein between its layers. The growth of the liver serves to bury this

free edge and the contained vein in a fissure on the caudal surface of

the liver. The same obtains in the case of the ductus venosus continued

from the umbilical vein (umbilical fissure and fissure of ductus venosus

of adult liver). Consequently the original continuity of the broad

ligament and lesser omentum, as parts of the primitive ventral

mesogastrium, is not readily seen in the adult.

The broad ligament extends across the convex cephalic surface of the

liver uniting it to the ventral abdominal wall and diaphragm, while its

free falciform edge apparently stops at the umbilical notch in the

ventral border of the organ. Actually, however, the obliterated vein is

surrounded in the bottom of the fissure, by a peritoneal fold which

effects the junction between broad ligament and lesser omentum.

We will see later in what way the permanent adult arrangement of the

lesser omentum is brought about. For the present we can state, on the

hand of the schematic Fig. 273, that the free caudal edge of the

falciform ligament containing the umbilical vein, and the free edge of

the gastro-hepatic omentum form together originally the caudal free edge

of the ventral mesogastrium, which membrane becomes separated, by the

growth of the liver, into suspensory or broad ligament and lesser or

gastro-hepatic omentum.

[Illustration: FIG. 275.--Abdominal viscera of _Iguana tuberculata_.

(Columbia University Museum, No. 1313.)]

This primitive disposition of the ventral mesogastrium and the viscera

connected with the same, is well shown in some of the lower vertebrates

in whom the development never proceeds beyond the early mammalian

stages. Fig. 275 shows in profile view from the right side the situs

viscerum and peritoneum in _Iguana tuberculata_.[5] The two dorsal

aortic roots are seen to unite to form the main aorta, which descends

between the layers of the dorsal mesentery, sending branches to the

dorsal margin of oesophagus and stomach. From the opposite border of the

stomach the ventral mesogastrium is derived. Its dorsal segment

(gastro-hepatic omentum) connects liver and stomach, carrying between

its layers the portal vessels, hepatic artery and biliary duct. The

ventral segment of the membrane, forming the suspensory or broad

ligament, extends between abdominal wall and ventral surface of the

liver. Caudad, the lesser omentum and the suspensory ligament are seen

to have a common concave falciform edge.

[5] _Iguana tuberculata_, one of the large lizards native of South

America. This animal forms an excellent object for the comparative study

of the visceral and vascular anatomy of the abdomen. It possesses a

well-differentiated intestinal tract, several coils of small intestine,

a well-marked caecum and large intestine. The examination of this or a

similar reptilian form is to be highly recommended. Iguana is easily

obtained in any of our large cities, as a considerable number of these

animals are annually imported from Mexico and the South American states.

The ventral abdominal vein ascends between the layers of the suspensory

ligament and near the liver becomes connected by a large branch with

the portal vein. A few smaller branches are seen passing from the

abdominal wall beyond this point. In this reptile, therefore, the

permanent vascular arrangement corresponds to an early human embryonic

stage.

The reptilian ventral abdominal vein is the homologue of the umbilical

vein of the placentalia. The large branch passing to the portal vein

represents the connection established in the human embryo between the

umbilical and portal veins. The small branches, continuing cephalad

between the mesogastric layers, represent the temporary proximal

remnants which in the human embryo the umbilical veins form in

connection with abdominal walls. The permanent adult arrangement of this

part of the vascular system in this animal corresponds therefore to one

of the stages of development in the human embryo, as previously

indicated (cf. p. 149; Figs. 251 and 252).

PERITONEAL RELATIONS OF LIVER.

It is well to begin the study of the peritoneal connections of the liver

with the consideration of the embryonic stage shown in Fig. 273

schematically.

If we imagine this embryonic liver detached from its connections in such

a manner as to leave the divided peritoneal layers of the ventral

mesogastrium as long as possible, and if we regard the preparation from

behind, the appearance of the parts could be represented in Fig. 276.[6]

[Illustration: FIG. 276.--Schematic view of embryonic liver detached

from its connections, seen from behind, with lines of peritoneal

reflection.]

[6] I am indebted to Dr. J. A. Blake, former Assistant Demonstrator of

Anatomy at Columbia University, for the valuable suggestion which led to

the preparation of Figs. 276, 277 and 278 together with the correlated

text.

It will of course be seen that the area of direct adhesion to the

diaphragm, extending transversely, would separate the lesser omentum

from the suspensory ligament.

As is seen in the transection (Fig. 274), the right and left layers of

the suspensory ligament, at its attachment to the liver, turn into the

visceral peritoneum investing the organ on its ventral and cephalic

surfaces. Continuing around the borders of the liver this visceral

peritoneum then invests in like manner the dorsal or caudal surface

directed toward the stomach, until, at the region of the future portal

or transverse fissure, this visceral peritoneum becomes in turn

continuous with the two layers of the lesser or gastro-hepatic omentum.

Consequently in the embryonic detached liver the lines of peritoneal

reflection would be nearly cruciform, the vertical limb of the cross

being formed on the cephalic surface by the two layers of the suspensory

ligament, while on the caudal surface it is formed by the layers of the

lesser omentum. The horizontal arm of the cross is formed by the upper

and lower limits of the area of diaphragmatic attachment, along which

the parietal diaphragmatic peritoneum turns into the visceral hepatic

investment (forming the two layers of the primitive coronary ligament).

In the liver shown thus schematically from behind we would overlook the

dorsal and adjoining portions of the cephalic and caudal surfaces of the

adult human liver.

The primitive biliary duct, portal vein and hepatic artery reach the

liver between the layers of the lesser omentum. The venae revehentes

(hepatic veins) reach the sinus venosus at the attachment of the liver

to the septum transversum (primitive diaphragm).

The first important change, resulting in a rearrangement of these

peritoneal layers, is produced by the connection of the umbilical with

the rudimentary portal vein.

This junction occupies a relatively wide area on the caudal surface of

the liver, and the layers of the lesser omentum are separated somewhat

at this point to accommodate the enlarging vascular structures between

them. More especially is this the case with the right leaf of the

primitive gastro-hepatic omentum. A species of lateral diverticulum is

formed by this leaf so as to include the umbilical vein at its junction

with the portal. The membrane in the region of this diverticulum turns

its surfaces dorsad and ventrad, and its free edge toward the right

(Fig. 277). With the gradual increase in the size of the vessels, and

with the transverse position which the rotation of the stomach imparts

to the opposite border of the lesser omentum attached to the lesser

curvature, this transversely disposed portion gradually exceeds in

length and size the part of the original omentum enclosing the umbilical

vein. This vessel and the investing peritoneum become lodged in a

sagittal depression on the caudal surface of the liver (rudimentary

umbilical fissure), while the transverse portion, developed as

indicated, surrounds the structures connected with the liver at the

future transverse or portal fissure.

[Illustration: FIG. 277.--Schematic view of embryonic liver, showing

influence of vascular connections on the arrangement of the lines of

peritoneal reflection.]

[Illustration: FIG. 278.--Later stages, showing development of

transverse fissure, Spigelian and caudate lobes.]

Schematically this rearrangement of the hepatic peritoneal lines of

reflection can be shown in Fig. 278.

It will be observed that in this way a small part of the caudal surface

of the right lobe has become partially marked off from the remainder as

a rudimentary Spigelian lobe, bounded ventrally by the transverse

fissure and lesser omentum attached to the same; to the left by the two

layers of the lesser omentum containing the ductus venosus; while the

limit cephalad is afforded by the reflection of peritoneum from liver to

diaphragm, forming part of caudal layer of right coronary ligament. To

the right this rudimentary Spigelian surface is directly continuous with

the rest of the dorsal and caudal surface of the right lobe (Fig. 277).

Finally a definite right limit is given to the Spigelian lobe by the

increasing size of the postcava and its closer connection with the

liver. This vessel now assumes the position of the main venous trunk

entering the heart from below.

This inclusion of the vena cava in the fissure or fossa of that name on

the dorsal surface of the liver affords, so to speak, the vertical

measure of the non-peritoneal area of the liver attached directly to the

diaphragm. As the vein develops the interval between the two layers of

the right coronary ligament increases, producing the well-known large

non-peritoneal area on the dorsal surface of the adult liver, which is

directly attached to the diaphragm.

Immediately to the left of the vena cava, however, the original

condition persists. The area of direct diaphragmatic attachment is

narrow and consequently the two layers of the coronary ligament are

close together at this point.[7]

[7] It should be remembered that in the final adult arrangement of the

abdominal viscera the liver shifts relatively backwards, so that the

diaphragmatic attachment, originally directed cephalad, now looks dorsad

and forms part of the dorsal or "posterior" surface of the adult organ.

The original ventral surface looks cephalad, as well as ventrad, forming

the convex surface which in the adult rests in contact with the

abdominal wall and diaphragmatic vault, while the surface originally

directed dorsad toward the stomach finally in large part has an

inclination caudad forming the "inferior" surface of human anatomy.

In this way a species of recess (Spigelian recess or hepatic antrum of

lesser sac) is formed. A portion of the dorsal liver surface lying just

to the left of the vena cava, between it and the ductus venosus, remains

invested by peritoneum which is reflected from the boundaries of this

space to the diaphragm. This forms the Spigelian lobe (Fig. 278).

The lobe is bounded to the right by the postcava, to the left by the

reflection of the lesser omentum to the stomach along the fissure for

the ductus venosus; cephalad the boundary is formed by the reflection of

the caudal layer of the coronary ligament to the diaphragm.

The caudal boundary is afforded by the transverse position which the

lesser omentum has assumed in the region of the transverse or portal

fissure.

It will be seen that the original continuity of the Spigelian lobe with

the caudal surface of the right lobe is maintained by the narrow bridge

of liver tissue connecting the caudal right angle of the rectangular

Spigelian lobe with the right lobe. This narrow isthmus, situated

between vena cava dorsad and the free right edge of lesser omentum

ventrad, forms the so-called _caudate lobe_.

[Illustration: FIG. 279.--Liver of human foetus at eighth month. View of

caudal and dorsal surfaces. (Columbia University Museum, No. 1854.)]

Fig. 279 shows a human foetal liver at the end of the eighth month in the

view from below and behind. The original continuity of the layers of the

lesser omentum, attached along the fissure for the ductus venosus, with

the fold of the falciform ligament occupying the umbilical fissure can

still be made out for a short distance beyond the left extremity of the

transverse fissure. The section of the lesser omentum which occupies the

transverse fissure and, including the portal vein, hepatic artery and

duct between its layers, terminates in the free right margin, is

evidently derived by a lateral extension from the right layer of the

primitive sagittal lesser omentum, whose original direction is preserved

along the fissure of the ductus venosus.

In Fig. 280 the lines of peritoneal reflection on the cephalic, dorsal

and caudal surfaces of a human foetal liver at term are shown.

[Illustration: FIG. 280.--Human foetal liver at term, showing lines of

peritoneal reflection on cephalic, dorsal, and caudal surfaces.

(Columbia University Museum, No. 1855.)]

We can now proceed to trace the reflection of the peritoneum from the

liver to adjacent structures.

Begin with the caudal layer of the coronary ligament on the extreme

right, where fusion with the corresponding cephalic layer produces the

right triangular ligament. The caudal layer of the coronary ligament

proceeds from right to left along the caudal margin of the

non-peritoneal dorsal diaphragmatic surface of right lobe, being

reflected along this line from the liver to the adjacent portions of the

diaphragm and ventral surface of right kidney and suprarenal capsule

(hepato-renal ligament). A small cephalic part of ventral surface of

right suprarenal capsule lies above this line of reflection, is hence

non-peritoneal and firmly connected with the liver just to the left of

entrance of vena cava into the caval fissure. Continuing, the caudal

layer of the coronary ligament crosses the ventral surface of the vena

cava and turns, immediately to the left of the vein, at a right angle,

ascending to form the left boundary of the Spigelian recess, being

reflected along this line from the left margin of the caval fissure to

the pillars of the diaphragm. Arrived at the opening of the central

tendon permitting passage of vena cava into pericardium, and at the

level of the entrance of the left hepatic vein into the cava, the

peritoneum turns again at a right angle and runs from right to left,

forming the cephalic limit of the Spigelian recess. Turning caudad along

the fissure for the ductus venosus, as right leaf of that portion of the

lesser omentum which is attached to this fissure and has preserved its

sagittal position, the peritoneal line of reflection reaches the left

extremity of the portal or transverse fissure. It now turns to the

right following the fissure as the dorsal layer of the transverse

segment of the lesser omentum, and becomes continuous, with the

formation of a free right edge, with the ventral layer of the same

membrane, passing from right to left, the two layers including between

them the structures entering and leaving the liver at the transverse

fissure (portal vein, hepatic artery, duct). Arriving at the left

extremity of the transverse fissure the ventral layer of the transverse

segment of the lesser omentum--as we practically trace it in the adult

as a free membrane--turns directly into the left leaf of the sagittal

segment attached along the fissure for the ductus venosus, and becomes

continuous along the dorsal border of the left lobe with the caudal

layer of the left coronary ligament. This direct continuity, as just

stated, exists practically in the adult. From the development of the

membrane, however, it will be seen that the ventral layer of the

transverse lesser omentum, at the left extremity of the portal fissure,

becomes continuous with the right layer of the primitive mesogastrium

enclosing the umbilical vein. After surrounding this vein it is

continued into the left leaf of the same membrane, which in turn passes

into the left layer of the portion attached along the fissure for the

ductus venosus.

This original connection can at times be traced very clearly in young

specimens (Fig. 279), and occasionally is also still evident in the

adult liver.

Usually, however, the round ligament of the adult and its investing

peritoneum is buried so deeply in the umbilical fissure, or even bridged

over in part by liver tissue, that the connection is not evident. The

ventral layer of the transverse omentum then appears directly continuous

with the left layer of the sagittal omentum attached along the fissure

for the ductus venosus.

We can sum up the facts just considered as follows:

1. The rotation of the stomach from the sagittal into the transverse

position, and the development of the umbilical and portal veins,

rearrange the original sagittal plane of the lesser omentum, dividing it

into two districts:

(_a_) Cephalic portion, remaining in the original sagittal plane,

follows the fissure for the ductus venosus. With the incorporation of

the Spigelian lobe in the adult dorsal or "posterior" surface of the

liver, this segment of the omentum assumes a vertical direction, forming

the left boundary of the Spigelian recess, being reflected from the

fissure for the ductus venosus to the abdominal portion of the oesophagus

and the part of the lesser curvature of stomach adjacent to the cardia.

(_b_) Distal caudal portion of the lesser omentum is twisted laterally

and turned to the right by the change in the position of the stomach and

the development of the structures connected with the liver at the

transverse fissure. It is reflected from this fissure to the distal part

of the lesser curvature and to the first portion of the duodenum. This

transverse segment of the lesser omentum is a secondary derivative from

the right leaf of the primitive membrane, produced by the enlarged area

for entrance of umbilical and portal veins at the transverse fissure. It

lies ventrad of caudal border of Spigelian lobe.

2. The distal segment of the original omentum containing the umbilical

vein (round ligament), continues imbedded in the umbilical fissure, to

the ventral margin of the liver, where it joins the layers of the

suspensory ligament passing over the cephalic surface.

3. The adult lesser omentum at the transverse fissure may be regarded as

a diverticulum of the right leaf of the primitive embryonal sagittal

omentum.

With the reduction of the umbilical vein after birth to form the round

ligament this structure becomes deeply buried in the umbilical fissure.

The ventral and dorsal layers of the lesser omentum at the transverse

fissure thus become continuous with respectively the left and right

layers of the second segment of the omentum which ascends vertically

along the fissure for the ductus venosus.

4. The cephalic layer of the coronary ligament (Fig. 280) remains

practically in the embryonic condition. The adult convex cephalic

surface of the liver is traversed in the sagittal direction by the

suspensory ligament which connects it with the abdominal surface of the

diaphragm, and thus effects the division into right and left lobes on

the convex surface. Arrived at the dorsal border of this surface

(junction of "superior" and "posterior" surfaces) the right and left

leaves of the falciform ligament turn at right angles into the cephalic

layer of the right and left coronary ligament, which at each extremity

meet the right and left caudal layers to form the triangular ligaments.

It will thus be seen that the apparent irregularity in the relative

arrangement of the s. c. "upper" and "lower" layers of the coronary

ligaments, produced by the Spigelian recess, is only a difference in the

interval between the two layers, caused by the vertical extent of the

non-peritoneal direct diaphragmatic attachment of the right lobe to the

right of the vena cava.

=Comparative Anatomy of Spigelian Lobe and Vena Cava in the Cat.=--The

lines of peritoneal reflection in the _cat's_ liver and the arrangement

of the Spigelian lobe and recess are seen in Fig. 281, taken from a

preparation hardened in situ.

[Illustration: FIG. 281.--Liver of cat, hardened _in situ_. (Columbia

University Museum, No. 1836.)]

Compared with the human liver it will be noted that the area of

diaphragmatic adhesion is much less developed. The dorsal surface of the

right lobe to the right of the postcava is peritoneal, there being no

extension laterad of the right coronary and triangular ligaments. The

postcava enters the liver in a special prolongation of the liver

substance (caval lobe).

The boundaries of the Spigelian recess and the lines of attachment of

the gastro-hepatic omentum correspond to the human arrangement.

RELATION OF THE HEPATIC PERITONEUM TO THE "LESSER SAC."

_Foramen of Winslow._--We have previously seen that the rotation of the

stomach and the further growth of the dorsal mesogastrium lead, in the

first instance, to the formation of the "lesser peritoneal cavity." This

cavity is in fact primarily the retrogastric space created by the

transverse position of the stomach, augmented by the cavity of the

omental bursa developed from the dorsal mesogastrium.

We have now to consider the additional boundaries of this space

contributed by the peritoneal connection of the lesser curvature with

the liver.

The lesser omentum follows, of course, along its gastric attachment to

the lesser curvature the general direction of the stomach, passing from

the cardia transversely downwards and to the right. We distinguish the

two layers of the adult membrane as ventral and dorsal, which meet in

the free right edge and include between them the main structures

entering and leaving the liver at the transverse fissure, viz.: the

portal vein, hepatic artery and bile-duct.

The lesser omentum therefore prolongs the plane of the stomach cephalad

towards the liver and thus forms the continuation of the ventral

boundary of the lesser peritoneal sac. We can now consider the line of

its hepatic attachment in the light of the facts previously adduced, and

combine the same with the line of gastric attachment to the lesser

curvature. Fig. 282 shows the foetal liver and stomach in their relative

position in the dorsal view, and Fig. 283 gives the lines of the

peritoneal reflections. The vertical segment of the omentum, occupying

the fissure for the ductus venosus, passes to the cardiac part of the

lesser curvature, its ventral layer covering the ventral and left side

of the oesophagus, while its dorsal layer passes to the dorsal and right

side of the oesophagus at its entrance into the stomach. The transverse

segment of the omentum, attached on the liver to the portal or

transverse fissure, accedes to the pyloric part of the lesser curvature.

Of course the ventral and dorsal layers of the omentum are continuous

with the serous visceral investment of the ventral and dorsal surfaces

of the stomach.

[Illustration: FIG. 282.--Dorsal view of human liver and stomach in

foetus at term, showing lines of hepatic and gastric attachment of lesser

omentum.]

[Illustration: FIG. 283.--Schema of lines of reflection of peritoneum on

dorsal surface of liver and in the formation of the gastro-hepatic

omentum. _A B_, transverse section of lesser omentum attached to

transverse fissure of liver and to pyloric section of lesser curvature

(_A' B'_); _B C_, vertical section of lesser omentum passing between

fissure of ductus venosus and cardiac section of lesser curvature of

stomach (_B' C'_); _C D_, line of reflection of peritoneum from cephalic

border of Spigelian lobe to diaphragm; _D E_, line of reflection of

peritoneum from right border of Spigelian lobe to left margin of

postcava and diaphragm.]

[Illustration: FIG. 284.--Portion of abdominal viscera of adult human

subject, hardened _in situ_. (Columbia University, Study Collection.)

The segment of stomach between cardiac and pyloric orifices has been

removed, dividing the lesser omentum to this extent, but leaving the

right extremity of the membrane (lig. hepato-duodenale) intact. Behind

this portion the arrow passes through the foramen of Winslow.]

[Illustration: FIG. 285.--Liver and stomach of _Macacus pileatus_.

(Columbia University, Study Collection.)]

Fig. 284 shows this right-angled course of the lesser omentum at the

hepatic line of attachment in a preparation of the abdominal viscera

hardened in situ, with the segment of the stomach between the cardiac

and pyloric orifices removed. The arrow is passed behind the right free

edge of the lesser omentum. This portion of the membrane is still

intact, not having been disturbed by the removal of the body of the

stomach, and includes between its layers the structures connected with

the liver at the transverse fissure (duct, hepatic artery and portal

vein). The lesser omentum is seen to be attached to the liver along the

transverse fissure (Fig. 284, _A_) and along the fissure for the ductus

venosus (Fig. 284, _B_), constituting the transverse and vertical

segments above referred to, which pass into each other at the angle of

junction between the transverse fissure (left end) and the fissure for

the ductus venosus (Fig. 284, _C_). The caudal and left border of the

Spigelian lobe is exposed by the division of the omentum, and the extent

of the Spigelian or hepatic recess of the lesser peritoneal sac is

shown. Fig. 285 shows the liver, stomach and lesser omentum of a Macaque

monkey hardened in situ, and demonstrates still more conclusively that

the uniform curve of the omentum along the lesser curvature of the

stomach becomes a broken line at the hepatic attachment, the angle being

placed at the left end of the transverse fissure at the point where the

same encounters the fissure for the ductus venosus.

[Illustration: FIG. 286.--Abdominal viscera of adult human subject,

hardened _in situ_; with liver lifted up after incision of the

gastro-hepatic omentum. (Columbia University Museum, No. 1845.)]

In Fig. 286 finally the hardened abdominal viscera of an adult human

subject are shown in the ventral view with the lesser omentum incised.

The cut through the lesser omentum exposes the hepatic recess of the

lesser peritoneal cavity immediately to the left of the foramen of

Winslow. Toward the right free margin of the omentum the divided portal

vein, hepatic artery and duct are seen between the layers of the omentum

imbedded in the pancreas and coursing behind the first portion of the

duodenum on their way to the transverse fissure.

To the left of these structures the omental tuberosity of the pancreas

projects above the level of the lesser curvature under cover of the

secondary parietal peritoneum forming the dorsal wall of the lesser sac,

while the lower edge of the Spigelian lobe appears in the upper angle of

the incision.

If we remember that the liver is itself welded to the diaphragm between

the layers of the coronary ligament (Fig. 280), it will become

apparent that the serous surface of the Spigelian lobe forms part of the

ventral wall of a peritoneal recess situated behind the lesser omentum,

between this membrane and the diaphragm. Access to this recess, without

the division of peritoneal layers, can only be obtained by passing from

right to left, along the caudate lobe, between the vena cava behind,

covered by parietal peritoneum, and the free right edge of the lesser

omentum in front. (In the reverse direction of the arrow shown in Fig.

284.) This hepatic or Spigelian recess of the lesser peritoneal cavity

has categorically the following boundaries (Figs. 282 and 283):

Dorsal: Parietal peritoneum, reflected along the line CD, from the

caudal layer of the coronary ligament to the diaphragm.

Ventral: Visceral peritoneum investing the Spigelian lobe and the

gastro-hepatic omentum.

Right: Reflection of peritoneum along the line DE (caval fissure) to

become the parietal peritoneum covering the diaphragm.

Left: Right layer of lesser omentum, reflected along the fissure for the

ductus venosus (CB) to the cardiac portion of the lesser curvature,

continuous with the dorsal layer of the lesser omentum reflected from

the transverse fissure to the pyloric segment of the lesser curvature

(AB).

We will presently see that certain relations of the vessels connected

with the liver at the transverse fissure and of the duodenum prevent the

finger, when passed from right to left behind the free right edge of the

lesser omentum and along the caudate lobe of the liver, from proceeding

downward at this point. A narrow channel of communication is thus formed

between the Spigelian recess and rest of the lesser sac on the one hand,

and the general greater peritoneal cavity on the other. This channel is

the so-called foramen of Winslow.

Having once passed this narrow space the finger will be in the Spigelian

recess and can palpate its boundaries. Further progress cephalad and to

the right is barred by the diaphragmatic adhesions of the liver just

detailed. But in the direction downward behind the lesser omentum and

along the dorsal surface of the stomach, as well as to the left toward

the spleen the excursion is limited only by the length of the examining

finger.

After opening the abdominal cavity of the human adult, elevating the

liver and depressing the stomach, the hepatic attachment of the lesser

omentum can be traced as already described. It will then be observed

that the gastric attachment of the membrane lies in one plane following

the lesser curvature while the hepatic attachment forms a broken line,

with the angle situated at the left extremity of the transverse fissure.

The vertical segment of the hepatic attachment, occupying the fissure

for the ductus venosus, turns at this angle into the transverse segment

which follows the transverse fissure to its right extremity where the

two layers pass into each other around the right free omental margin

(hepato-duodenal ligament). Consequently we overlook, in an abdominal

cavity thus exposed, the entire caudal surface of the liver, including

the caudal surfaces of right, left, and quadrate lobes. The junction of

right and caudate lobes can be seen between vena cava and right edge of

the omentum, or rather, it can be felt at this point. But the Spigelian

lobe, turning its surface dorsad against the parietal peritoneum

covering the diaphragm, forms part of the "posterior" liver surface and

is not visible, although--as just stated, it can be palpated by passing

the finger through the foramen of Winslow. The Spigelian lobe cannot be

overlooked in its entire extent until the liver is removed from the body

and regarded from behind. The caudal edge (continuation of its right

angle into the caudate lobe and papillary tubercle) can be seen by

tearing through the layers of the lesser omentum and lifting the liver

up forcibly (Fig. 286).

=Caudal Boundary of Foramen of Winslow.=--We have above referred to the

fact that the finger introduced through the foramen of Winslow meets in

this canal with resistance if an attempt is made to pass downwards.

After passing this constricting point the free excursion into the

Spigelian recess and behind the omentum and stomach and toward the

spleen can be performed.

In considering the elements which produce this narrowing of the

communication between the two peritoneal sacs at the foramen of Winslow

we have to deal with two factors, one primary and constant, the other

secondary and inconstant.

1. The first of these is afforded by the arrangement of the arterial

vessel supplying the liver. The hepatic artery is a branch of the coeliac

axis, furnishing arterial blood to the liver tissues and supplying, in

addition, branches to the stomach, duodenum and pancreas.

This vessel is, of course, placed primarily, like all other arterial

branches supplying the alimentary tract, between the layers of the

primitive dorsal mesentery. Originally the vessel supplies the distal

(pyloric) portion of the stomach along its dorsal attached border

(subsequently the greater curvature) corresponding to the adult

gastro-epiploica dextra of the hepatic (gastro-duodenalis).

It likewise gives branches to the adjacent pyloric portion of the

duodenum and the pancreas, as that gland develops from the intestine,

corresponding to the adult superior pancreatico-duodenal branch, and to

the ventral border (lesser curvature) of stomach, corresponding to the

adult pyloric branch of the hepatic.

With the development of the liver from the duodenum arterial branches

derived from this primitive gastro-duodenal vessel pass to the sprouting

hepatic cylinders by continuing around the duodenum, beneath its serous

investment, to reach the interval between the two layers of the ventral

mesogastrium, in which the liver develops, near the free margin of this

membrane.

After the rotation, which turns the right side of the stomach, duodenum

and mesoduodenum dorsad, the branch which passes over the dorsal surface

of the duodenum to reach the liver becomes more favorably situated and

develops into the main hepatic artery which reaches the liver at the

transverse fissure between the folds of the lesser omentum. The original

right side of the duodenum, now turned dorsad, adheres to the parietal

peritoneum. The hepatic artery which reached the liver by passing over

this surface of the duodenum, beneath its visceral serous covering,

becomes imbedded in connective tissue by the adhesion of the visceral

duodenal and the primitive parietal peritoneum. Hence in the adult the

hepatic artery courses imbedded in the connective tissue which binds the

duodenum to the abdominal background to reach the interval between the

two omental layers which carry it to the transverse fissure.

The hepatic artery, therefore, derived from one of the primitive

intestinal branches (gastro-duodenal) is, notwithstanding its hidden

position in the adult, originally situated between the layers of the

free primitive dorsal mesogastrium.

It now becomes necessary to regard the development of the great omentum

from the primitive dorsal mesogastrium in relation to this course of the

hepatic artery. We have seen that the great omentum and the cavity of

the omental bursa is produced by the extension of the dorsal

mesogastrium to the left and caudad, subsequent to the rotation of the

stomach. The splenic artery and the left gastro-epiploic branch pass

from the coeliac axis to the left between the layers of the mesogastrium,

as previously seen (Figs. 291 and 292).

The hepatic artery, however, is so to speak placed on the border line

between the portion of the primitive mesentery which, as dorsal

mesogastrium, is to turn to the left and caudad to form the great

omentum, and the portion which, as mesoduodenum, turns to the right and

passes to the duodenal loop (Fig. 287).

[Illustration: FIG. 287.--Primitive dorsal and ventral mesogastrium with

course of hepatic artery.]

[Illustration: FIG. 288.--The liver divides the ventral mesogastrium

into a dorsal segment, the gastro-hepatic or lesser omentum, and a

ventral segment, the suspensory or falciform ligament of the liver.]

[Illustration: FIG. 289.--Stages in the development of the dorsal

mesogastrium (omental bursa) and mesoduodenum to show relation of

hepatic artery to these two segments of the primitive common dorsal

mesentery.]

In the further course of development the dorsal mesogastrium grows more

and more, forming the omental bag, while the mesoduodenum on the other

hand becomes anchored early and obliterated as a free membrane by

adhesion of its original right layer to the primitive parietal

peritoneum. The hepatic artery runs on the line dividing these two

different mesenteric segments. We can imagine, so to speak, that the

redundant growth of the omentum to the left and caudad, takes place over

the hepatic artery as a resistant support (Figs. 288 and 289). Cephalad

of the hepatic artery is the developing omentum, caudad of the vessel

the mesoduodenum. The artery follows the cephalic limit of the

mesoduodenum and becomes, as stated, adherent to the abdominal

background in the segment between its origin from the coeliac axis and

the point where, after having crossed the dorsal surface of the

duodenum, it enters the right edge of the lesser omentum on its way to

the liver.

=Pancreatico-gastric Folds.=--If we open the lesser peritoneal cavity by

dividing the gastro-hepatic omentum and look into the background of the

retro-omental space, we will see a fold of the secondary lining parietal

peritoneum (derived from the mesogastrium), which can be traced from the

cephalic border of the pancreas to the pyloric extremity of the stomach.

This fold carries the hepatic artery to the lesser omentum behind the

first portion of the duodenum, and is called the right or main

pancreatico-gastric fold. A similar fold, further to the left, carries

in a like manner the coronary artery of the stomach to the cardiac end

of the lesser curvature. This fold forms the left or secondary

pancreatico-gastric fold. Between the two folds the caudal margin of the

Spigelian lobe projects into the lesser cavity.

The appearance of the two pancreatico-gastric folds in the adult human

subject is well seen in Fig. 284.

[Illustration: FIG. 290.--Abdominal viscera of _Nasua rufa_, brown

coaiti, with stomach turned up and great omentum divided. (From a fresh

dissection.)]

Fig. 290 shows the abdominal cavity of _Nasua rufa_, with great omentum

divided to bring into view the vessels passing from coeliac axis to liver

and stomach and elevating the retrogastric parietal peritoneum to

produce the pancreatico-gastric folds.

(The course of the hepatic artery from coeliac axis to liver in the

dorsal view in the cat is seen in Fig. 223.)

[Illustration: FIG. 291.--Schematic transection through foramen of

Winslow before adhesion of dorsal mesogastrium and mesoduodenum to

parietal peritoneum.]

[Illustration: FIG. 292.--The same section after the adult conditions

have been established by adhesion.]

Figs. 291 and 292 represent schematically cross-sections directly

through the foramen of Winslow, showing the method by means of which the

hepatic artery reaches the upper border of the duodenum and the effect

of the adhesion of duodenum and mesoduodenum upon the disposition of the

vessel.

The coronary artery, like the splenic, is at first situated between the

layers of the dorsal mesogastrium (vertebro-splenic segment). Like the

splenic the coronary artery becomes anchored to the abdominal background

and placed secondarily behind the parietal peritoneum of the lesser sac

by the adhesion of this mesogastric segment to the primitive parietal

peritoneum. To reach the lesser curvature at the cardia and to run

thence from left to right along the lesser curvature between the layers

of the gastro-hepatic omentum, the vessel raises the investing parietal

peritoneum (originally the right leaf of the dorsal mesogastrium) into a

crescentic fold, extending between its origin from the coeliac axis at

cephalic margin of pancreas and the beginning of the lesser curvature of

the stomach. Hence this fold is called the left pancreatico-gastric

fold. (Seen well in Fig. 284.)

In the next place it must be borne in mind that the relation of the

primitive hepatic artery to the vascular supply of the stomach, pancreas

and duodenum produces a permanent shortening of the primitive mesentery

at this point. This result is indicated in the schematic figures 287,

288 and 289.

In the original condition the dorsal mesentery, passing to a practically

straight intestinal tube, is of uniform sagittal measure (Fig. 287).

As development proceeds, and as the liver grows from the duodenum, the

hepatic artery develops from the primitive pyloric vessel as above

indicated. This vessel, assuming greater importance with the rapid

growth of the liver, is not lengthened out as happens with the remaining

purely intestinal branches which follow the increase in the length of

the intestinal canal. The hepatic artery, therefore, will mark the point

where the original short sagittal extent of the primitive mesentery will

tend to be preserved. Cephalad of this point the dorsal mesogastrium

grows out into the great omentum (Figs. 288 and 289); caudad of the same

point the membrane, in following the development of the intestine,

becomes drawn out into the permanent mesentery and mesocolon.

The hepatic artery, in addition, marks the cephalic limit of the

adhesion which anchors the duodenum and mesoduodenum to the parietal

peritoneum. Consequently in the adult the vessel courses in as direct a

manner as possible, taking the shortest course from the coeliac axis to

the liver, passing dorsad of the duodenum and giving what now appear as

secondary branches to supply the intestine, the stomach and pancreas

(pyloric and gastro-duodenal arteries (pancreatico-duod. superior and

gastro-epiploica dextra)).

Even if no fixation of the duodenum and mesoduodenum takes place this

course of the hepatic artery will produce a constricted passage between

the liver (caudate lobe) cephalad, abdominal parietes and aorta dorsad,

lesser omentum and pyloric duodenum ventrad, and hepatic artery caudad.

This passage leading from the general peritoneal cavity into the

retrogastric space is the _primitive foramen of Winslow_. This condition

is well represented in the abdominal cavity of some of the lower

mammalia, in which duodenum and mesoduodenum remain permanently free.

Fig. 293 shows a view of the abdominal cavity from the right side in a

specimen of the ant-eater, _Tamandua bivittata_.

[Illustration: FIG. 293.--Abdominal viscera of _Tamandua bivittata_, the

little ant-eater, with the intestines turned downward and to the left.

(From a fresh dissection.)]

The right kidney is seen in the background, covered by the parietal

peritoneum. The duodenum and mesoduodenum are free and can be turned

toward the median line. The opening of the foramen of Winslow leading

into the retrogastric space is seen between the liver cephalad, kidney

and vena cava dorsad, lesser omentum and pyloric extremity of the

stomach ventrad, and a fold of peritoneum carrying the hepatic artery

caudad. Exactly similar conditions prevail in the cat and in many other

mammals.

It will be seen in all these instances that neither portal vein nor

bile-ducts limit the foramen caudad. These structures can be lifted up

and turned toward the median line with the free duodenum and

mesoduodenum. But the hepatic artery must pass to the liver from the

_retroperitoneal coeliac axis_. In doing this the vessel traverses the

cephalic border of the pancreas, and the pyloric extremity of the

stomach and duodenum, to reach the lesser omentum which conveys it to

the liver.

Consequently there must always be a narrow peritoneal neck between the

liver cephalad, aorta dorsad, hepatic artery caudad, and pyloric

extremity of stomach and duodenum together with the lesser omentum

ventrad. It should be remembered that the vessel which extends after the

development of the liver into the lesser omentum as the _hepatic_

artery, was originally destined for the supply of these latter

structures. In the adult these primary embryonic terminal branches to

the intestine appear as secondary branches derived from the hepatic as

the main vessel. Their origin, however, serves to keep the beginning of

the small intestine in comparatively close connection with the hepatic

artery which courses over the dorsal surface of the duodenum to reach

the liver. The narrow space thus left between aorta, hepatic artery,

duodenum, lesser omentum and liver forms the framework of the foramen of

Winslow and appears always as a confined and narrow channel. This

relation is shown in the accompanying schematic Figs. 294 and 295 which

represent a sagittal section through the foramen. This primitive foramen

is thus bounded cephalad by the liver (caudate lobe, connecting

Spigelian and right lobes), ventrad by the first portion of the duodenum

and the lesser omentum, with hepatic artery behind the intestine and

between the omental layers; dorsad by the abdominal background and large

retroperitoneal vessels, and caudad by the coeliac axis and beginning of

the hepatic artery.

[Illustration: FIG. 294.--Schematic sagittal section through foramen of

Winslow before fixation of pancreas by adhesion of mesoduodenum.]

[Illustration: FIG. 295.--The same section after adhesion of

mesoduodenum and pancreas. The pancreas appears secondarily

retroperitoneal, after adhesion of apposed surfaces of mesoduodenum and

primitive parietal peritoneum over dotted area, producing fixation of

dorsal surface of pancreas.]

2. In the forms which possess in the adult an adherent duodenum and

mesoduodenum, as in man, the foramen of Winslow obtains a secondary

caudal limit by the agglutination of the descending duodenum and the

parietal prerenal peritoneum. This is the secondary and inconstant

factor referred to above in the caudal boundary of the foramen. The

result of this anchoring of duodenum and mesoduodenum is to bring the

margin of the foramen further to the right and to bury the hepatic

artery still further from view. Thus in the adult human subject the

structures bounding the foramen at the margin of the entrance into the

narrow channel would be above caudate lobe of liver, behind postcava,

below duodenum adherent to ventral surface of right kidney, in front

first portion of duodenum and lesser omentum. The hepatic artery will be

felt on introducing the finger through the foramen in its original

position, but it will be seen that the actual boundaries of the foramen

have been moved so to speak a little further to the right by the

duodenal adhesion.

[Illustration: FIG. 296.--Dissection of adult liver, pancreas, spleen,

and duodenum, with vessels, to show structures concerned in the

formation of the foramen of Winslow. (Columbia University, Study

Collection.)]

Fig. 296 shows a complete dissection of the adult human viscera and

vessels concerned in the formation of the foramen, hardened in situ.

The stomach is removed, dividing of course the coronary artery and vein

and the left gastro-epiploic artery. The portal vein, hepatic artery and

bile-duct are seen entering and leaving the liver at the transverse

fissure. Behind them and to the right the vena cava enters the liver.

The hepatic artery distributes its pancreatico-duodenal branches to the

duodenum and pancreas. The left angle of the Spigelian lobe and the

fissure for the ductus venosus appear to the left of the portal vein and

hepatic artery. The right angle of the Spigelian lobe and its

continuation into the right lobe by means of the caudate lobe is hidden

by the structures occupying the transverse fissure. We would enter the

beginning of the foramen of Winslow by passing between the vena cava

behind, the structures in the transverse fissure (portal vein, hepatic

artery and duct) in front, caudate lobe of liver above and duodenum

below, the latter in the undisturbed condition of the parts adherent to

the right kidney. Continuing to the left the finger would pass between

aorta behind, coeliac axis and hepatic artery below and in front, and

liver above. These structures bound the permanent and primary narrow

channel of communication between the retrogastric or lesser peritoneal

space and the general peritoneal cavity, which exists even if a free

duodenum and mesoduodenum allow us to lift the intestine away from vena

cava and right kidney.

The main facts pertaining to the structure of the lesser peritoneal sac

and its connection with the greater peritoneal cavity by means of the

foramen of Winslow may be summed up as follows:

The mesogastrium as a whole, expanding originally in the sagittal plane

in a fan-shaped manner between the vertebral column and the ventral

abdominal wall, from the level of the umbilicus to the septum

transversum (diaphragm), divides the cephalic part of the abdominal

cavity into a symmetrical right and left half.

Figs. 172 and 273 represent the membrane as seen in a profile view from

the left side. We distinguish the segment dorsad of the stomach as the

dorsal mesogastrium, directly continuous with the remaining segments of

the common primitive dorsal mesentery, while the portion ventrad of the

stomach forms the ventral mesogastrium in which the liver develops. The

segment of the ventral mesogastrium between liver and stomach becomes

the lesser or gastro-hepatic omentum, while that between liver and

ventral abdominal wall forms the falciform or suspensory ligament.

A transection, showing the dorsal and ventral mesogastrium at the level

of the fundus of the stomach, is given in Fig. 298. The mesogastria are

here seen to be short, while in the schematic Figs. 291 and 292 the

membrane is, for the sake of distinctness, represented as being of

considerable extent.

[Illustration: FIG. 297.--Schematic sagittal section of the ventral and

dorsal mesogastria and epiploic bursa in a human embryo of eight weeks.

(Modified from Kollmann.)]

[Illustration: FIG. 298.--Transection of human embryo of 3 cm.,

vertex-coccygeal measure. (Kollmann.)]

The ventral mesogastrium surrounding the liver and stomach extends

caudad to include the first portion of the duodenum. Beyond this point

it terminates in a thickened free edge which includes the umbilical

vein. This vein extends from the umbilicus to the transverse fissure of

the liver (Fig. 297), lying within the umbilical fissure on the caudal

surface of the gland.

At the point where the vein enters the liver the thickened margin of the

ventral mesogastrium is continued, as ligamentum hepato-duodenale, to

the upper part of the duodenum and forms the ventral boundary of the

foramen of Winslow. Between the layers of the mesogastrium which meet in

this margin are situated the portal vein, biliary duct and hepatic

artery, together with the nerves and lymphatics of the liver.

The mesogastrium originally divided the abdominal cavity between

umbilicus and diaphragm into symmetrical right and left halves of equal

size and extent. This early symmetrical arrangement becomes disturbed

about the seventh week by the rotation of the stomach and the resulting

altered course of the mesogastrium, which render the two original equal

halves of the abdominal cavity unequal and asymmetrical. The original

right half becomes placed behind the stomach and is converted into a

blind sac with its opening directed to the right.

The communication of the general abdominal cavity with the retrogastric

space by means of this channel is still wide in the embryo, but

gradually becomes narrowed in the course of further development to form

the foramen of Winslow. This opening is situated between the

hepato-duodenal ligament and the parietal peritoneum covering the vena

cava. It is constricted from below by the curve of the hepatic artery as

this vessel passes from the coeliac axis to reach the liver at the

transverse fissure between the layers of the lesser omentum.

The earlier developmental stages of the higher mammalian embryos are in

general well illustrated by the permanent adult conditions found in some

of the lower vertebrates, in which development does not proceed beyond

the primitive condition.

In reptiles, birds and mammals the epiploic bursa is generally formed,

while in amphibia the dorsal mesogastrium is very short and connects the

stomach directly to the dorsal midline of the abdominal cavity without

forming the sac-like extension of the great omentum.

The dorsal mesogastrium with the stomach, and the ventral mesogastrium

including the liver between its layers, divides in these animals the

cephalic part of the body cavity into two halves, corresponding to the

earlier embryonic stages in man and in the higher mammalia.

The foramen of Winslow of the higher forms appears in the lower

vertebrates as the wide-open space leading from below into the right

half of the coelom cavity. The dorsal mesogastrium remains short, not

forming the pouch-like extension of the great omentum. The stomach

retains more or less its primitive vertical position without rotation or

elevation of the pyloric extremity, and the intestinal canal is simple,

short and comparatively straight.