1792 



HUMAN ANATOMY. 



t-rif^' 



^Acinus 



of secretion and the distention of the acini. Their Uning consists of a single layer of 

 fairly regular polygonal cells, about .010 mm. in diameter, the height of the cells 

 varying with the dilatation of the follicle. In young subjects, in whom the acini are 

 generally less completely tilled than in older ones, the epithelium of the follicles 

 approaches the columnar type. A similar condition is often to be noted in certain 

 acini, even in thyroids in which the usual distention affects the majority of follicles. 

 A distinct basement membrane is wanting, the cells resting directly upon a somewhat 

 condensed stratum of the surrounding connective tissue. Since the epithelial lining 

 is the source of the peculiar colloid secretion of the gland, the cells ordinarily con- 

 tain a variable number of highly refracting granules, particularly in the zone next 

 the sac. The peculiar substance or colloid commonly found within the follicles of 

 the adult organ is regarded as a proteid, although its exact chemical characteristics 

 are still uncertain. The consistence of this substance varies, being more fluid in 

 young than in old glands. Its varying appearance within the follicles, as vacuo- 

 lated, reticular, or shrunken, is referable to the action of reagents, in its natural 

 condition the secretion being homogeneous and entirely filling the follicle. The 

 differentiation of the epithelial lining of the acini into chief and colloid cells (Lang- 

 endorff), as representing distinct elements, is doubtful, since specific differences 

 probably do not exist. 



Vessels. — The blood-supply is very generous, coming from two pairs of rela- 

 tively large arteries, the superior thyroids from the external carotids, and the in- 

 ferior thyroids from the 

 Fig. 1 513. subclavians. The superior 



descend to the top of the 

 lateral lobes and ramify 

 over the front of the organ, 

 sending branches to the 

 interior, and sometimes 

 meeting on the isthmus. 

 The inferior arteries pass 

 upward behind and enter 

 the organ on its inner sur- 

 face. Their relations to 

 the inferior laryngeal nerve 

 are of practical impor- 

 tance. In 437 observa- 

 tions ' the artery was found 

 in front of the nerve on 

 the right in about 41 per 

 cent, and on the left in 

 63 per cent. In over 10 

 per cent, of the cases the 

 branches were so inter- 

 laced that the relation was 

 uncertain. It is evident 

 that in enlargement of the 

 thyroid body, with conse- 

 quent enlargement of the arteries, the number of such indefinite relations would be 

 very much increased, as very minute branches would then spring into importance. 

 An enlarged tortuous artery tends to curl around the nerve. There was no artery on 

 the right in one case and none on the left in five cases of this series. An arteria 

 thyroidea ima springing from the arch of the aorta and ascending in the median line is 

 occasionally seen. From the rich superficial arterial plexus numerous branches pass 

 along the interlobular septa, following the ramifications of the latter to the follicles, 

 where the arterioles break up into capillaries. These surround the follicles with close- 

 meshed net-works, which are often common to the adjacent sacs, resembling the 

 capillary net- works around the pulmonary alveoli. 



The vei?is are very numerous. Emerging from the organ, they form a large 



' Dwight : Anatom. Anzeiger, Bd. x., 1895. 



Interlobular 

 vessels 



Capillary 

 net-work 

 surrounding 

 acinus 



Acinus con- 

 taining col- 

 loid 



Section of injected thyroid body. X 46. 



