THYROID GLAN1 



1269 



THE DUCTLESS GLANDS. 



There are certain organs which are very similar to secreting glands, but differ 

 from them in one essential particular, viz., they do not possess any ducts by which 

 their secretion is discharged. These organs are known as ductless glands. They 

 are capable of internal secretion that is to say, of forming, from materials brought 

 to them in the blood, substances which have a certain influence upon the nutritive 

 and other changes going on in the body. This secretion is carried into the blood 

 stream, either directly by the veins or indirectly through the medium of the 

 lymphatics. 



These glands include the thyroid, the parathyroids and the thymus ; the pituitary 

 body and the pineal body ; the chromaphil and cortical systems to which belong the 

 suprarenals, the paraganglia and aortic glands, the glcmus caroticum and perhaps 

 the glomus coccygeum. The spleen is usually included in this list and sometimes the 

 lymph and hemolymph nodes described with the lymphatic system. Other glands 

 as the liver, pancreas and sexual glands give off internal secretions, as do the 

 gastric and intestinal mucous membranes. 



External carotid artery 



Superior thyroid artery 

 Superior thyroid vein 



Vagus nerve 



Middle thyroid vein 



FIG. 1174. The thyroid gland and its relations. 



THE THYROID GLAND (GLANDULA THYREIODEA; THYROID BODY) 



(Fig. 1174). 



The thyroid gland is a highly vascular organ, situated at the front and sides of 

 the neck; it consists of right and left lobes connected across the middle line by a 

 narrow portion, the isthmus. Its weight is somewhat variable, but is usually about 

 30 grams. It is slightly heavier in the female, in whom it becomes enlarged during 

 menstruation and pregnancy. 



The lobes (lobuli gl. thyreoidece) are conical in shape, the apex of each being 

 directed upward and lateralward as far as the junction of the middle with the lower 





