200 



NORMAL HISTOLOGY. 



: 



Fibrous capsule .55 



separating -_' 



thyroid and /s" 



parathyroid * ^ 



Capillary 



FIG. 248 Section including adjacent portions of human thyroid 

 (above) and parathyroid (below). X 220. 



vessels. The distinctive tissue consists of closely placed polygonal epithelial 

 fells (10 AI in diameter), disposed as continuous masses or as imperfectly 

 separated cords and alveoli. The cells have round nuclei and are lodged 

 within a reticulum composed of wide capillaries and delicate strands of fibro- 



elastic tissue, the whole 

 often bearing a striking 

 likeness to the anterior lobe 

 of the pituitary body (page 

 309), even to the presence 

 of colloid substance within 

 some of the alveoli. When 

 the cell-masses tend tow- 

 ards the alveolar type, the 

 epithelium and the blood- 

 channels are in intimate 

 relation, an arrangement 



A ' P!" ^^ kcUitating the 



distribution of the partic- 

 ular product of the cells. 

 The significance of the 

 parathyroid bodies as dis- 

 tinct organs, and not as 

 merely masses of modified 

 thyroid tissue, has been 

 established ; in certain ani- 

 mals, their loss results in 

 tetany and death; like the 

 thyroid, they are ' ' ductless glands ' ' and organs of internal secretion. 

 The blood-vessels supplying the organs are the minute parathyroid 

 arteries, usually from the branches of the inferior thyroid, to each one of 

 which a body is attached. The capillaries are relatively wide and ramify 

 between the nests of cells. Little is known concerning the lymphatics and 

 nerves; the latter, however, are chiefly sympathetic fibres for the walls of 

 the blood-vessels. 



THE THYMUS BODY. 



Although actually increasing in size and weight until towards puberty, 

 the thymus body is essentially an organ of very early childhood, attaining 

 its highest development about the second year. At that time it stretches 

 from the root of the neck downwards into the thorax, behind the sternum and 

 over and in front of the pericardium, to about the line of the fourth costal 

 cartilage. It is thickest above and descends as two flattened irregular lobes, 

 separated by fibrous tissue, of which the left one is more often the larger. 

 Subsequently more or less extensive atrophy and replacement of the thymus- 

 tissue occurs, variable islands of the latter surrounded and invaded by fat- 

 cells being the usual condition of adolescence. Notwithstanding this replace- 

 ment by adipose and connective tissue, the thymus never entirely disappears, 

 remains of its tissue being present even in extreme old age. 



The thymus body develops from paired epithelial outgrowths from the 

 ventral wall of the third pharyngeal furrows. From these result long cylin- 

 drical masses of closely packed epithelial cells, which grow downwards and 

 for a time enclose a lumen that later disappears. The masses increase by 

 solid outgrowths, resembling those of an immature tubo- alveolar gland, so 



