The Thyroid Gland and its Diseases 



Anatomy, Embryology, Comparative 

 Anatomy and Histology 



E. V. COWDRY 



NEW YORK 



Anatomy 



Gross Morphology and Relations. The thyroid gland is a reddish 

 yellow, tabulated, U-shaped structure, divisible into right and left lateral 

 lobes, which are joined together in the median line by an isthmus. It 

 is named after the thyroid cartilage, so-called on account of its shield-like 

 shape (Greek, 0ypeds, shield, 1 and cZ5o>, form). 



The thyroid is situated in the middle third of the neck and partially 

 encloses the trachea dor sally (Fig. 1). Its ventral surface is covered by 

 the infrahyoid muscles, the cervical fascia, and the skin. It is in relation 

 dorsally with the first two to four rings of the trachea, the cricoid cartilage, 

 esophagus, and pharynx, the common carotid arteries, vagus nerves, and 

 other structures. It is held in place by a fairly strong capsule connected 

 with the cervical fascia. This capsule is said to be divisible into two 

 layers, an outer, which can be removed with ease, and an inner one, which 

 is very adherent to the gland. 



The isthmus is frequently absent, and even when it is present it is 

 often difficult to palpate, owing to fibrosis. Occasionally in the normal 

 condition even the gland itself cannot be felt at all. The right lobe is 

 often a little larger than the left. The lobes measure about six centimeters 

 in length. A so-called pyramidal process often extends cephalad from the 

 isthmus, usually on the left side. It may even reach as far as the foramen 

 cecum at the base of the tongue. It consists of typical thyroid tissue. In 

 very rare cases it may be double, one on either side of the middle lint. 

 Accessory thyroids usually occur near the hyoid, but they may be found 

 as far caudad as the bifurcation of the trachea. The weight of the gland 

 is very variable, a good average being about 35 grams. Variations of as 

 much as 20 grams are not to be considered abnormal. In some locali- 

 ties, depending upon altitude, climate, and other factors, it is normally 

 larger than in others (Table 1). It is normally relatively one^fourth 



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