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LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



The relations of the mesenteries are very similar in all of the vertebrates. The dorsal 

 mesentery extends, usually unbroken, from the dorsal median line to the digestive tract. 

 Each portion of it bears a special name, according to the part of the digestive tract which it 

 supports. In mammals the mesentery of the stomach is drawn out posteriorly into a bag, the 

 greater omentum, which apparently serves to protect the abdominal viscera (Fig. 50). The 

 ventral mesentery extends from the digestive tract to the ventral body wall but is reduced to 

 remnants in the adults of all vertebrates. These remnants are found in the region of the 

 liver and the bladder where they form the gastrohepatic and hepatoduodenal ligaments, the 

 falciform ligament, and the median ligament of the bladder. All viscera of vertebrates either 

 are retroperitoneal, i.e., situated external to the coelomic lining, or they are situated between 

 the two layers of a mesentery. The liver, the heart, and the urinary bladder are situated in 

 the ventral mesentery; the pancreas is generally in the dorsal mesentery but may be in the 

 ventral mesentery as well; the digestive tube may be regarded as situated in the dorsal 

 mesentery. 



Fig. 50. — Diagrams to show the formation of the greater omentum in mammals and the fusion 

 of the mesogaster and the mesocolon. A, early stage, in which the mesogaster is beginning to form a 

 bag at g. B, the mesogaster is drawn posteriorly into a long bag g, which is the greater omentum; 

 the mesogaster and mesocolon are fusing at i. C, completion of the fusion of mesogaster and mesocolon 

 ati. u, liver; b, serosa of the liver; c, lesser omentum or gastro-hepa to-duodenal ligament; d, stomach; 

 e, lesser peritoneal sac or cavity of the greater omentum; /, mesocolon; g, portion of the mesogaster 

 which forms the greater omentum; h, intestine; ?', fusion of the mesogaster and mesocolon. (From 

 Prentiss and Arey's Textbook of Embryology, after Hertwig, courtesy of the W. B. Saunders Company.) 



I. SUMMARY 



1. The parts of the digestive tract are much the same in all of the vertebrates. There has 

 been little evolution of this system. In general, the intestine is longer and more coiled in the 

 higher forms. 



2. The mouth cavity is provided with glands in land vertebrates. 



3. The nasal cavities are blind sacs in fishes, but beginning with Amphibia they com- 

 municate with the mouth and pharyngeal cavities for respiratory purposes. At first these 

 respiratory passages through the nose open into the anterior end of the mouth cavity but later 

 a hard palate, a shelf of bone, is developed ventral to the nasal passages. By this means the 

 internal nares are moved posteriorly. In birds and mammals the nasal passages are extended 

 still farther posteriorly by the development of the soft palate; the air then passes directly 

 into the pharyngeal cavity without first passing the mouth cavity. 



4. The wall of the pharynx is pierced by gill slits provided with gills in fishes and without 

 gills in urodeles. Above the urodeles the gill slits vanish, with the exception of the first which 

 contributes to the external auditory meatus. The first visceral pouch evaginates to form the 



