75U COMPARATIVE ANATOMY 



they pushed forth by a double layer of serosa which form their respec- 

 tive mesenteries. This serous covering is continuous with the covering 

 of the entire intestinal tract and is known as the visceral peritoneum 

 which also forms the lining of the abdominal cavity. 



The liver is the largest gland in the body, but no matter how many 

 lobes it may develop or how large it may grow, a layer of serosa covers 

 every part of the gland except that part lying toward the side from 

 which it grew. Here, as stated, the layers coming from each side nat- 

 urally unite and form the double layer of serosa the mesentery. The 

 two large suspensory mesenteries of the liver are called the ligamentum 

 hepato-gastricum (sometimes also called the lesser omentum) and the 

 ligamentum suspensorium-hepatis. 



Practically the entire length of the digestive canal which passes 

 through the body-cavity is attached by both a dorsal and ventral mesen- 

 tery. The ventral mesentery becomes lost below the region of the liver, 

 leaving a sharp ventral edge to the two hepatic ligaments. 



The function of the liver is to secrete bile (gall), as well as to form 

 various internal products such as glycogen, urea, and uric acid, all of 

 which substances are of great importance to the living animal. The 

 bile is sent to the intestines through the bile duct (also called choledochal 

 duct), while the other products are carried away by the blood. Sub- 

 stances which are not sent through a duct but are carried throughout 

 the body by the blood stream are known as substances of internal secre- 

 tion. 



The liver is a compound tubular gland. The many little tubules in 

 the liver which form the gall capillaries empty into the bile duct. This 

 tubular condition of the liver is easily seen in ichthyopsida, but is dim- 

 cult to observe in mammals because of the tubular anastomosis and be- 

 cause of the close interrelation of the bile vessels and blood vessels. 



The liver begins its growth cephalad, at about the same time the 

 blood vessels have already developed into the large sinus venosus and 

 hepatic veins. These blood vessels also contribute to the septum trans- 

 versum (Fig. 348). The growth of these latter organs prevents the liver 

 from continuing the cephalad growth so that from now on it increases 

 in size in an opposite direction. 



Concomitant with its increase in size there is an immigration of 

 mesenchyme between the lobules of the liver. The blood vessels enter 

 at this time. The bile duct (if there are several, this is only true of one 

 of them) has a lateral diverticulum or enlargement. This is the gall 

 bladder (Figs. 426, II, 436), which serves as a reservoir for the bile. It 

 may be found in the substance of the liver itself, but is usually more or 

 less separate and lies dorsal to the liver substance. It is lacking in some 

 mammals. In fact, it is not uncommon in man to have it removed sur- 

 gically. 



Both the liver and the gall bladder have ducts leading from them. 



