Biology of Man 507 



sex hormones may be present in certain annelids and Crustacea. Color- 

 influencing hormones which affect pigment cells have been studied experi- 

 mentally in Crustacea. Some endocrine glands, such as testes, ovaries, 

 and pancreas, may function as both ductless and duct glands. Some 

 endocrine glands, such as the pituitary, thyroid, parathyroids, and 

 adrenals, function only as ductless glands. Several of them produce a 

 number of hormones with more or less specific functions, which compli- 

 cates the problem of investigating them. 



P\nQa\ 



-V Pituitary 



Parathyroids^ 



.Thyroid 



Thymus 



Liver \—m^ 



—ifcomacb 

 .-Spleen 



-y^drcnalj 

 -Pancreas 

 _ htestinQ 



Ovaries 



Testes 



Fig. 252. — Approximate locations of endocrine (ductless) glands of human being. 



The location and brief descriptions of some of the human endocrine 

 glands are as follows: 



1. Pituitary (Hypophysis). — Small (1 cm. diameter), reddish-gray; 

 located at center of the base of the brain; composed of an anterior lobe, 

 intermediate portion, and a posterior lobe which is connected with the 

 hypothalamus of the brain; in certain animals the two lobes are sep- 

 arate (Fig. 252). 



2. Thyroid. — A pair of shield-shaped glands connected by an isthmus 

 and located in front of the trachea, just below the larynx; present in all 

 vertebrates; thyroxin is an amino acid containing iodine. 



