THE ENDOCRINE SYSTEM 



617 



half an ounce, but have a richer supply of blood vessels per mass of 

 tissue than any other organ of the body. Each adrenal consists of two 

 parts, an outer, pale, yellowish-pink cortex and a dark, reddish-brown, 

 inner medulla. In cyclostomes and fishes the two parts are spatially 

 separate; in amphibians, reptiles and birds their anatomic relations are 

 quite variable and the two parts are interspersed. Cortical tissue de- 

 velops from coelomic mesoderm near the mesonephric kidneys, whereas 

 the medullary tissue is ectodermal, derived from the neural crest cells 

 which also form the sympathetic ganglia. 



The cells of the medulla are arranged in irregular cords and masses 

 around the blood vessels (Fig. 30.5). The medulla secretes two closely 

 related hormones, epinephrine (also called adrenin and adrenaline) and 

 norepinephrine. These are comparatively simple chemicals derived 

 from the amino acid tyrosine. Epinephrine produces an increase in heart 

 rate, a rise in blood pressure, a decrease in liver glycogen and an increase 

 in blood glucose. It causes dilation of the pupils of the eye, gooseflesh 

 and dilation of most blood vessels but constriction of those of the skin, 

 so that the skin becomes pale. Norepinephrine has much weaker effects 

 on blood sugar and heart rate but is a more powerful vasoconstrictor. 



cotnxt 



ton* 

 CLOJueKULOtA 



ZONA 

 FASCICULATA 



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 RCTICt/LARtS " 



ycetra 



Mteuua 



Figure 30.5. Sections through the adrenal cortex and medulla of normal (A) 

 and hypophysectomized (B) rats. (Turner: General Endocrinology.) 



