THYROID GLAND. 



1105 



confirming the remark, that the nuclei, instead 

 of remaining in their primitive condition, pro- 

 ceed to the further stage of cell development ; 

 this he has noticed both in man and in several 

 of the lower animals. I should say that it is 

 certainly a circumstance of rather rare oc- 

 currence ; but it is worth remarking, that it 

 may be artificially produced by adding to the 

 specimen some coagulating re-agent, which 

 speedily solidifies a film of albuminous plasma 

 around the nuclei, and thus produces very 

 good imitations of cells. The epithelium of 

 the thyroideal cavities often assumes the form 

 of small vesicles larger than the nuclei (see 

 Jigs. 736, 737, 738.), and easily distinguished 



Fig. 736. 



o. 



Fig. 737. 



Vesicular epithelium 

 from Human thyroid. 

 Diameter of vesicles, 



3_20ou m - 



Versicular epithelium 

 from thyroid of Pig. 

 Diameter of vesicles, 



7m HI. 



Fig. 738. 



Vesicle from thyroid of Hedgehog, lined by an 

 epithelium consisting of a double row of pellucid 

 delicate vesicles, r in. diarn. 



it is, however, certain, that they are not de- 

 veloped upon pre-existing nuclei. The layer 

 of epithelium is generally of no great thick- 

 ness, not occupying more than one eighth or 

 one sixth of the distance from the envelope 

 to the centre of the cavity ; in the rabbit, 

 however, it appeared to be more abundant, 

 encroaching considerably on the interior, 

 which, in this instance, was not filled with the 

 characteristic "-listening secretion. In a sec- 



O O 



tion prepared in the ordinary way, a large 

 quantity of epithelium is broken up, and may 

 be seen strewn over the field. Not infre- 

 quently, however, the nuclei adhere firmly 

 together; and sometimes, as in the pig, I have 

 seen the greater part of the lining of a cavity 

 detached entire. 



The contents of the cavities are for the 

 most part a clear, somewhat refracting, homo- 

 genous material, which is manifestly the pro- 

 duct of secretion, and fills all the spaces not 

 occupied by the epithelium : this fluid is some- 

 times contained in small vesicles ^2 to ir i 7T 

 inch diam. (Jigs. 739. and 740.) which have a 



Fig. 739. 



from them.* The diameter of these in a hu- 

 man subject averaged *_ inch; in a bullock, 

 about T7 Vo mcn They are, in their natural 

 state, perfectly spherical, but often somewhat 

 angular from mutual pressure. Their contents 

 are a very faintly granular or pellucid material, 

 which does not surround a nucleus except iu 

 some rare instances, where there may be seen 

 an imperfect trace of one. These vesicles, 

 which I thus name to distinguish them from 

 the nucleated cells occasionally met with, exist 

 in the glandular cavities, sometimes alone, 

 sometimes mingled with the ordinary form of 

 epithelium in varying proportion. I am in- 

 clined to believe that they originate in the 

 nuclei, which undergo a kind of expansion, 

 at the same time losing their nucleoli. This 

 opinion needs further confirmatory evidence; 



* I am obliged to use the same word (vesicle) 

 in speaking of the large glandular cavities, and of 

 the epithelial particles which line them : the dis- 

 tinction between them should, however, be carefully 

 borne in mind. 

 VOL. IV. 



O 



Two vesicles containinq a transparent matter and no 

 epithelium. From thyroid of Bullock; the larger 

 5^ in., the smaller in. diam. 



Fig. 740. 



Two I'csicles from thyroid of Bullock, having a 

 thicker lining of epithelium than usual, and each 

 containing a single vesicle, whose wall, of homo- 

 geneous membrane, surrounds the central cavity of 

 the original vesicle. 



well marked, structureless envelope, but are 

 destitute of any thing like epithelium. They 

 may be seen occasionally in the interior of the 

 glandular cavities, and also floating free in the 

 field of view, having been perhaps detached 

 from cavities opened by the section. The 

 exact import of this circumstance does not 

 appear ; for I cannot regard them as newly 

 formed glandular vesicles, developed within 

 the original ones in an endogenous manner. 

 Were this the case they would occur more 

 frequently, and would exhibit some' traces of 

 epithelial" lining. Large crystals, sometimes 



4 u 



