THE THYROID GLAND. 1123 



in the function of respiration. It is situated at the upper part of the trachea, and 

 consists of two lateral lobes, placed one on each side of that tube and connected 

 together by a narrow transverse portion, the isthmus. 



Its anterior surface is convex, and covered by the Sterno-hyoid, Sterno-thyroid, 

 and Omo-hyoid muscles. 



Its lateral surfaces, also convex, lie in contact with the sheath of the common 

 carotid artery. 



Its posterior surface is concave, and embraces the trachea and larynx. The 

 posterior borders of the gland extend as far back as the lower part of the pharynx, 

 and on the left side to the oesophagus. 



The thyroid varies in weight from one to two ounces. It is larger in females 

 than in males, and becomes slightly increased in size during menstruation. Each 

 lobe is somewhat conical in shape, about two inches in length, and three-quarters 

 of an inch to an inch and a quarter in breadth, the right lobe being the larger of 

 the two. 



The isthmus connects the lower third of the two lateral lobes; it measures 

 about half an inch in breadth and the same in depth, and usually covers the second 

 and third rings of the trachea. Its situation presents, however, many variations 

 a point of importance in the operation of tracheotomy. Sometimes the isthmus 

 is altogether wanting. 



A third lobe, of conical shape, called the pyramid, occasionally arises from the 

 upper part of the isthmus or from the adjacent portion of either lobe, but most 

 commonly the left, and ascends as high as the hyoid bone. It is occasionally quite 

 detached, or divided into two parts, or altogether wanting. 



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 glandules 

 thy roiJ < i':. 



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

 which projects into its substance and imperfectly divides it into masses or lobules 

 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 Dr. Baber, who has recently published some important observa- 

 tions 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 

 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 run in the interlobular connective tissue, not uncommonly surrounding the 

 arteries which they accompany, and communicate with a network in the capsule 



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



