THE DUCTLESS GLAITOS 



These glands do not possess excretory ducts. They furnish 

 materials which are added to the blood or Ijmiph as it passes through 

 them. The material from each gland is known as an internal se- 

 cretion, or hormone. Some of these secretions are powerful ma- 

 terials and influence profoundly the body nutrition. The ductless 

 glands are usually given as follows: the spleen, the lymph glands, 

 the pineal gland, the pituitary body, the thyroid gland, the thymus 

 gland, the adrenal glands, and the parathyroids. The spleen, the 

 pituitary, the pineal, and the lymph glands are described in other 

 sections. 



The Thyroid Gland (Fig. 21, No. 16). Location. — The thyroid 

 gland hes on the ventral side of the carotis communis at a point 

 where the carotis communis touches the jugular vein, which is about 

 the point of origin of the vertebral artery. 



Shape. — The thyroid gland is small, oval or somewhat roundish, 

 and red or rose-colored. 



Structure. — The thyroid gland has a fibrous capsule, which sends 

 into the interior septa which divide it into acini. These acini are 

 closed and contain a fluid. The thyroid is a ductless gland. Short 

 arteries from the carotis enter this gland, and some large veins con- 

 nect it with the jugular vein. The lymph vessels whicji lie along the 

 neck are closely connected with it and receive twigs from the gland. 

 The minute lymphatic capillary endings are found in its septa and in 

 its capsule. The acini are lined with a single or a double row of 

 cuboidal secreting cells. There are two kinds of cells, namely, 

 secreting and resting cells. The actively secreting cells secrete 

 colloid. 



The parathyroids consist of two smaU bodies attached to the 

 lower pole of the thyroid. 



The Thymus Gland (Fig. 47, No. 10).— The thymus glaiid is an 

 organ of fetal and early baby chickhood. It soon undergoes retro- 

 gressive changes into fat and connective tissue. It is of epithelial 



190 



