THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 



161 



pharynx 



2. The differentiation of the digestive tube. Along its course, the tube soon differentiates 

 into various regions with different functions. These regions, beginning at the anterior end, are: 

 mouth or oral cavity, pharynx, esophagus, stomach, small intestine, large intestine or colon, 

 cloaca. The esophagus may be provided with an enlargement, the crop, as in birds. The 

 stomach may be subdivided into two or more compartments, each with special functions, as in 

 birds and cud-chewing mammals. The first part of the small intestine is named the duodenum; 

 the remainder, if of sufficient length, is subdivided into jejunum and ileum. There is generally 

 a blind pouch, the caecum (sometimes more than one), at the junction of small and large intes- 

 tine. The colon may be divided into several regions. Its terminal portion is often named 

 the rectum. The rectum passes into 



the cloaca which opens to the exterior 

 by the anus. The cloaca receives not 

 only the intestine but also the genital 

 and urinary ducts. 



3. The outgrowths of the diges- 

 tive tube. A number of outgrowths 

 arise from the tube at various levels 

 (see Fig. 46). 



a) The oral glands: The mouth 

 cavity is commonly provided with 

 glands which consist of evaginations 

 of the lining epithelium. These glands 

 are of two kinds: mucous glands, 

 which secrete a slimy fluid used to 

 moisten the mouth cavity and the 

 food and in some forms as an aid in 

 capturing prey; and salivary glands, 



visceral pouch 

 gill slit 

 thyroid gland 



stomach 



duodenum 



pancreas 



liver 



yolk sac 



small intestine 



urinary bladder 



anterior lobe 

 of hypophysis 



oral cavity 



cloaca 



FIG. 46. Diagram to illustrate the chief derivatives of 

 the digestive tract. (From McMurrich's Development of 

 the Human Body, after His, copyright by P. Blakiston's 

 Son and Company.) 



characteristic of mammals, in which 

 the secretion contains, not only mucus, 

 but also digestive enzymes. The 

 names of the oral glands indicate 

 their positions in the walls of the 

 mouth cavity. 



b) The anterior lobe of the hypoph- 

 ysis: From the roof of the mouth 



cavity an evagination occurs in the embryo, producing a blind pouch which extends toward 

 and comes in contact with the floor of the brain in a certain region. This pouch, known as 

 the anterior lobe of the hypophysis, fuses with a portion of the brain wall, the compound struc- 

 ture thus formed being designated the hypophysis 01 pituitary body. It is one of the glands of 

 internal secretion. 



c) The thyroid gland: This is an outgrowth from the floor of the pharynx midway between 

 the second gill slits. It is homologous with the endostyle of lower chordates and is a gland of 

 internal secretion, producing a secretion necessary for health and growth. 



d) The visceral pouches and gills: The visceral pouches are paired evaginations from the 

 wall of the pharynx, typically five or six in number in vertebrates. Opposite each visceral 

 pouch, the ectoderm invaginates, forming the visceral furrow. Visceral pouches and furrows 

 meet at their extremities and the point of fusion breaks through, the opening being known as a 

 gill slit or visceral cleft (see Fig. 47). The entoderm lining the visceral pouch grows out into 

 thin plates or delicate filaments, the gill, used for respiration. The tissue between successive 

 visceral pouches and slits is called the visceral arch. In the center of each visceral arch is a 



