ORGANS OF SECRETION. 607 



bivorous tribes, and in some its place is supplied by a mere 

 dilatation of the hepatic duct, as in the horse and elephant, 

 and the same is seen in the dolphin ; it is most constant 

 and most developed in the carnivora ; it is sometimes double 

 or even triple as abnormal varieties ; and its fundus still 

 often receives several hepato-cystic ducts as in lower classes. 

 In the dolphin and grampus, where, as in most cetacea, 

 there is no gall-bladder, nor pyloric valve to mark the termi- 

 nation of the gastric cavities, the united duct from the liver 

 and pancreas opens directly into the muscular duodenal 

 dilatation, commonly termed the fifth stomach, and the dilated 

 reservoir supplying the place of gall-bladder in the dolphin, 

 as in the elephant, is an enlargement of the common trunk 

 of the hepato-pancreatic duct, receiving, like the duct of a 

 cephalopod, the secretions both of the liver and pancreas. 

 The vena portae forms a sinus in the dolphin before di- 

 viding in the liver, and in the diving seals and otters 

 the trunks of the hepatic veins are surrounded with a con- 

 tractile muscular tunic, which is immediately invested with 

 an exterior plexiform layer of the small returning anastomosing 

 branches of the intra-lobular hepatic veins, to allow of the 

 safe contraction of these muscular hepatic trunks. In the 

 echidna and ornithorhyncus the gall-bladder is present, along 

 with a dilatation of the common biliary duct, seen also in 

 several marsupialia, before piercing the duodenum ; the pan- 

 creatic and common choledochal ducts continue separate, 

 and have distant terminations, in the echidna, but these two 

 early unite in the ornithorhyncus, to form a common hepato- 

 pancreatic duct. 



The liver is proportionately large in the human embryo, 

 as in the lower vertebrata and the molluscous animals in 

 their adult condition. The follicular structure of the embryo- 

 liver was already observed by Malpighi in the chick, 

 before the end of the first week of incubation, and before 

 the end of the first month, the liver in the human embryo 

 has attained nearly half the weight of the entire body, and is 

 lobulated like the kidney of a bear, and extended backwards 

 to near the pelvis. The gall-bladder early makes its appear- 

 ance as a developing cylindrical tubulus, and gradually be- 

 comes more elongated, conical, hollow, rnuciparous, rugous 

 internally, and at length biliferous ; the liver becomes less 



