2 -g NORMAL HISTOLOGY. 



according to the size and the distention of the acini ; they usually con- 

 tain a viscid yellowish mass, the colloid substance, produced through 



the active agency of the cells lining the 

 FIG. 293. acini. In addition to the characteris- 



tic colloid secretion, detached epithe- 

 lium, leucocytes, migrated plasma- 

 cells, and in very many cases colored 

 blood-corpuscles, are included within 

 the contents of the alveoli. The pres- 

 ence of red blood-cells in various stages 

 of disintegration has suggested the 

 destruction of effete blood-cells as a 

 possible function, in part at least, of 



Section of thyroid body, exhibiting de- , .. 11 rri , 



tail of the acini! which are cut in various thlS questionable Organ. The inter- 

 directions : c, colloid material distending alveolar tissue contains elements closely 



the larger acini ;/. interacinous connective , , ,, 



tissue?,, biood-vesseis. resembling plasma-cells. 



The blood-vessels of the thyroid 



gland are exceptionally numerous, the arteries being remarkable for 

 their large size and very free anastomoses. From the larger 

 branches, which run within the interlobular tissue, small twigs pass 

 between the alveoli and break up into capillaries surrounding the 

 acini with a close-meshed net-work situated immediately beneath 

 their epithelium. The venous radicles are also numerous, and form 

 the conspicuous superficial plexuses. 



The plentiful lymphatics occupy the deeper connective-tissue 

 septa between the lobules as well as the fibrous envelopes surrounding 

 the lobes. The deeper lymphatics begin as spaces lying between 

 the bundles of fibrous tissue close to the acini, and frequently contain 

 characteristic colloid substance. Large superficial trunks, provided 

 with valves, carry off the accumulations from the smaller canals. 



The few nerves which supply the thyroid gland are derived almost 

 entirely from the sympathetic system. The fibres, therefore, are prin- 

 cipally of the pale, non-medullated variety, and seem to be distributed 

 especially to the walls of the blood-vessels ; a few medullated fibres are 

 usually present, but the exact mode of their termination is uncertain. 



The development of the respiratory organs begins as a ven- 

 tral evagination of the entodermic lining of the primitive pharynx. 

 The caudal extremity of this complex cavity abruptly narrows into 

 the cesophageal division of the primary gut tract. The earliest in- 

 dication of the formation of the respiratory apparatus consists in the 

 extension of the ventro-dorsal diameter of the primitive oesophagus 

 at its pharyngeal end, in which plane it now appears as an irregularly- 

 compressed ellipse. 



The pulmonary evagination extends caudally for some distance, 



