1104- 



THYROID GLAND. 



once have been considered that a gland should 

 exist unprovided with an efferent duct. 



I now proceed to give a more detailed 

 account of the structure of the thyroid gland. 

 Its surface is somewhat uneven, a natural 

 condition which is often greatly exagge- 

 rated in hypertrophy of the gland ; it is tra- 

 versed by several large branches of the 

 nutrient arteries, which ramify over it before 

 they plunge into its substance. A thin fibrous 

 expansion, continuous with the sheath of the 

 cervical vessels on each side, forms a capsule 

 which invests the gland, and from whose 

 inner surface septa dip into the interior, 

 dividing its substance into lobes and lobules 

 much after the manner of a conglomerate 

 gland ; these fibrous septa are often well seen 

 in sections of hypertrophied specimens. A 

 thin slice of the thyroid, examined under a 

 low power of the microscope, displays its con- 

 stitution very perfectly and readily (fig. 733.) 



It is seen to be made up of closed vesicles, 

 aggregated together in groups of various size 

 by the fibrous expansions just described. The 

 form of these vesicles is primarily spherical ; 

 but many, perhaps the majority, are more or 

 less affected by mutual pressure, being trian- 

 gular, elongated, ovoid, or oblong. They are 

 all perfectly closed, the wall being formed by 

 an homogeneous limitary membrane, which is 

 easily traced all round, and can never be seen 

 passing off into a neck, or blending with the 

 envelope of an adjacent vesicle. Where a 

 number of vesicles lie closely crowded to- 

 gether, the homogeneous envelopes are of 

 course in contact, or separated only by the 

 interjacent vascular plexus ; but those form- 

 ing the surface of a group are invested by 

 a thin expansion of fibrous tissue derived 

 from the general capsule. The diameter 

 of the vesicles of the human thyroid I have 

 found to range from -^^ inch to ^ 



Fig. 733. 



> 'A group of vesicles from the thyroid gland of the bullock. 



Iney contain the vesicular epithelium, and that consisting of nuclei and granular matter, in varying 



proportions. 



inch ; in the bullock, from ^Vo mc ^ to 

 & inch ; the greater number averaging 

 about ^ inch, in this animal as well as in 

 the pig. In the mesian column I found, at 

 least in one instance, that the structure was 

 essentially the same as that of the thyroid 

 itself, only that there was a much greater 

 amount of fibrous stroma, which resembled 

 more nearly ordinary areolar tissue, containing 

 both the white and yellow element. 



The vfisicles are lined internally by an epithe- 

 lial stratum, consisting usually of nuclei set 

 closely together in a scanty basis substance 

 (fig. 734.), which is either feebly granular, or of 

 a somewhat oily aspect. The nuclei are at 

 once recognised by the practised eye as 

 exactly resembling those of the true glands. 

 Their nucleoli are not always visible, and vary 



Fig. 734. 



Fig. 735. 



Epithel^mfrom thyroid Epithelial parties fro 

 J bl thyroid of rabbit 



Diameter^ in. 



very much in number from one to four or 

 five. The nuclei are, however, always vesicu- 

 lar, bounded by a strongly marked envelope 

 and have a mean diameter of ^^ th inch. 

 It has been observed by Mr. Simon, and I 

 have occasionally had the opportunity of 



