FORM AND STRUCTURE OF THE THYROID BODY. 



921 



and sometimes called the cornu, is usually connected to the side of the 

 thyroid and cricoid cartilages by areolar tissue. 



The transverse part, or isthmus, which connects the two lateral lobes to- 

 gether a little above their lower ends, measures nearly half an inch in 

 breadth, and from a quarter to three-quarters of an inch in depth ; it com- 

 monly lies across the third and fourth rings of the trachea, but is very 

 inconstant in size, shape, and position, so that the portion of trachea which 

 is covered by it is subject to corresponding variations. From the upper part of 

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

 the left, a conical portion of the thyroid body, named, from its shape and 

 position, the pyramid, or middle lobe, often proceeds upwards to the middle 

 of the hyoid bone, to which its apex is attached by loose fibrous tissue. 

 Commonly this process lies somewhat to the left ; occasionally it is thicker 

 above than below, or is completely detached, or is split into two parts : 

 sometimes it appears to consist of fibrous tissue only. In many cases, mus- 

 cular fasciculi, most frequently derived from the thyro-hyoid muscle, but 

 occasionally independent, descend from the hyoid bone to the thyroid gland 

 or its pyramidal process. They are known as the levator glanduloe thy- 

 roidece. It sometimes, though rarely, happens that the isthmus is alto- 

 gether wanting, the lateral lobes being then connected by areolar or fibrous 

 tissue only. 



The weight of the thyroid body varies ordinarily from one to two ounces. 

 It is always larger in the female than in the male, and appears in many of 

 the former to undergo a periodical increase about the time of menstruation. 

 The thyroid body, moreover, is subject to much variation of size, and is, 

 occasionally, the seat of enormous enlargement, constituting the disease 

 called goitre. The colour of the thyroid body is usually of a dusky brownish 

 red, but sometimes it presents a yellowish hue. 



Fig. 645. MAGNIFIED VIEW OF Fig. 6 in. 



SEVERAL VESICLES FROM THE 

 THYKOID GLAND OF A CHILD (from 

 Kolliker). *f 



a, connective tissue between the 

 vesicles ; 6, capsule of the vesicles ; 

 c, their epithelial lining. 



Structure. The texture of 

 this organ is firm, and to the 

 naked eye appears coarsely 

 granular. It is invested with 

 a thin transparent layer of 

 dense areolar tissue, which con- 

 nects it with the adjacent parts, 

 surrounds and supports the 

 vessels as they enter it, and 

 imperfectly separates its sub- 

 stance into small masses of 



irregular form and size. The interstitial areolar tissue is free from fat, and 

 contains elastic fibres. 



When the thyroid body is cut into, a yellow glairy fluid escapes from the 

 divided substance, which is itself found to contain multitudes of closed 

 vesicles, composed of a simple external capsular membrane, and containing 



