NUTRITION IN ANIMALS 



159 



Other glandular derivatives of the digestive tract in Man 

 are the SALIVARY glands? of the mouth, the THYROID and 

 THYMUS glands near the anterior end of the oesophagus, and 

 the GASTRIC and INTESTINAL glands imbedded in the wall of 

 the stomach and intestine respectively. As a matter of fact 



Duct 



Alveoli 



Secreting Cells 



Capillary Network 



FIG. 90. Diagram of a gland, in section, together with the surrounding connective 

 tissue and blood vessels. Highly magnified. (From Hough and Sedgwick.) 



the thymus degenerates, while the thyroid loses all connection 

 with the alimentary canal and contributes its products directly 

 to the blood. Accordingly the thyroid as well as a number of 

 other similar glands, are known as DUCTLESS, or ENDOCRINE, 

 glands, and their products as INTERNAL SECRETIONS. (Fig. 88.) 

 At first glance the complicated digestive system of the 

 Vertebrate may seem to have little in common with that of ^ ^ ; 

 the Earthworm, but as a matter of fact the fundamental plan 

 is the same. The differences which are present are the re- 

 sult of an increase of the working area of the alimentary 



