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THE ORGANS OF VOICE AND RESPIRATION. 



A few muscular bands are occasionally found attached, above, to the body of 

 the hyoid bone, and below to the isthmus of the gland or its pyramidal process. 

 These" form a muscle, which was named by Sommerring the Levator glandulce 

 thyroidce. 



Small detached portions of thyroid tissue (accessory thyroid] are sometimes 

 found above the isthmus, and their presence is readily explained by a reference to 

 the manner in which the gland is developed. They represent isolated portions of 

 the median thyroid rudiment. (See section on Embryology.) 



Structure. The thyroid body is invested by a thin capsule of connective tissue 

 which projects into its substance and imperfectly divides it into masses of irregular 

 form and size. When the organ is cut into, it is of a brownish-red color, and is 

 seen to be made up of a number of closed vesicles containing a yellow glairy fluid 

 and separated from each other by intermediate connective tissue. 



According to Baber, who has published some important observations on 

 the minute structure of the thyroid, 1 the vesicles of the thyroid of the adult 

 animal are generally closed cavities ; but in some young animals (e. g., young 



Vesicle. 



Lymphatic vessel. 



Wall of gland-vesicle. 



FIG. 540. Minute structure of thyroid. From a transverse section of the thyroid of a dog. (Semi-diagram- 

 matic.) (Baber.) 



dogs) the vesicles are more or less tubular and branched. This appearance he 

 supposes to be due to the mode of growth of the gland, and merely indicating that 

 an increase in the number of vesicles is taking place. Each vesicle is lined by a 

 single layer of epithelium, the cells of which, though differing somewhat in shape 

 in different animals, have always a tendency to assume a columnar form. Between 

 the epithelial cells exists a delicate reticulum. The vesicles are of various sizes 

 and shapes, and contain as a normal product a viscid, homogeneous, semi-fluid, 

 slightly yellowish material which frequently contains blood, the red corpuscles 

 of which are found in it in various stages of disintegration and decolorization, the 

 yellow tinge being probably due to the haemoglobin, which is thus set free from 

 the colored corpuscles. Baber has also described in the thyroid gland of the 

 dog large round cells (" parenchymatous cells "), each provided with a single oval- 

 shaped nucleus, which migrate into the interior of the gland-vesicles. 



The capillary blood-vessels form a dense plexus in the connective tissue around 

 the vesicles, between the epithelium of the vesicles and the endothelium of the 

 lymph-spaces, which latter surround a greater or smaller part of the circumference 

 of the vesicle. These lymph-spaces empty themselves into lymphatic vessels 

 which ran in the interlobular connective tissue, not uncommonly surrounding the 

 1 "Researches on the Minute Structure of the Thyroid Glands," Phil. Trans., part iii., 1881. 



