THE THYROID GLAND. 139 



trachea and upon the roots of the bronchia than elsewhere. 

 They are placed behind the muscular layer, which their excre- 

 tory ducts have to penetrate. Besides occupying these situa- 

 tions, they are found in the interstices between the cartilagi- 

 nous rings, but here they are much smaller. The mucous 

 membrane abounds so much in the orifices made by their ex- 

 cretory ducts, that it looks cribriform, which appearance is in- 

 creased by floating it in water. 



About the origins of the bronchia, there is a considerable 

 number of black coloured lymphatic glands, called bronchial, 

 which it is easy to distinguish from the preceding by their 

 colour and much greater size. 



There are two more glands of a different character, which, 

 though they do not enter into the composition of the trachea, 

 yet, from their locality, are most conveniently studied at this 

 time': they are the Thyroid and the Thymus. 



SECT. II. OF THE THYROID GLAND. 



The Thyroid Gland (Glandula Thyroidea) is placed on the 

 first and second rings of the trachea, and on the sides of the 

 larynx. 



It consists in a middle portion which is thin, of variable mag- 

 nitude in different individuals, sometimes entirely wanting, and 

 which, being stretched across the upper part of the trachea just 

 below the larynx, is called its isthmus: and of two lobes, one 

 on each side, which, being flattened and ovoidal, are extended 

 considerably upwards on the side of the larynx, and downwards 

 on the side of the trachea and of the oesophagus. Frequently 

 from the superior part of the isthmus, and most commonly on 

 its left side, a small pyramidal process runs upwards in front of 

 the cricoid and of the thyroid cartilage, and is attached, by liga- 

 mentous fibres to the os hyoides; this process, however, varies 

 much in size and length ; I have never seen it double. Accord- 

 ing to Morgagni and Meckel, its existence is much more com- 

 mon than its absence, which corresponds with my own obser- 

 vations. The thyroid gland, when extended, measures about 

 three inches from side to side. 



