788 ORGANS OF VOICE AND RESPIRATION. 



size during menstruation. It occasionally becomes enormously hypertrophied, 

 constituting the disease called bronchocele, or goitre. Each lobe is somewhat 

 conical in shape, about two inches in length, and three-quarters of an inch in 

 breadth, the right lobe being rather 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, aud 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 left side of the upper part of the isthmus, or from the left lobe, 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 pro- 

 cess. These form a muscle, which was named by Sommerring the Levator 

 Glandulse Thyroidese. 



Structure. The thyroid consists of numerous minute closed vesicles, composed 

 of a homogeneous membrane, inclosed in a dense capillary plexus, and connected 

 together into imperfect lobules, by areolar tissue. These vessels are spherical 

 or oblong, perfectly distinct, and contain a yellowish fluid, in which are found 

 floating numerous "dotted corpuscles" and cells. The fluid coagulates by heat 

 or alcohol, but preserves its transparency. In the foetus, and in young subjects, 

 the corpuscles lie in a single layer, in contact with the inner surface of these 

 cavities, and become detached during the process of growth. 



Vessels and Nerves. The arteries supplying the thyroid, are the superior and 

 inferior thyroid, and sometimes an additional branch (thyroidea media, t. ima) 

 from the arteria innominata, or the arch of the aorta, which ascends upon the 

 front of the trachea. The arteries are remarkable for their large size and 

 frequent anastomoses. The veins form a plexus on the surface of the gland, 

 and on the front of the trachea, from which arise the superior, middle, and 

 inferior thyroid veins; the two former terminating in the internal jugular, the 

 latter in the vena innominata. The lymphatics are numerous, of large size, and 

 terminate in the thoracic and right lymphatic ducts. The nerves are derived 

 from the pneumogastric, and from the middle and inferior cervical ganglia of 

 the sympathetic. 



Chemical Composition. The thyroid gland consists of albumen, traces of 

 gelatine, stearine, oleine, extractive matter, alkaline, and earthy salts, and 

 water. The salts are chloride of sodium, alkaline sulphate, phosphate of potash, 

 lime, magnesia, and a trace of oxide of iron. 



THYMUS GLAND. 



The Thymus Gland presents much resemblance in structure to other glandular 

 organs, and is another of the organs denominated ductless glands. 



The thyrnus gland is a temporary organ, attaining its full size at the end of 

 the second year, when it ceases to grow, and gradually dwindles, until, at 

 puberty, it has almost disappeared. If examined when its growth is most 

 active, it will be found to consist of two lateral lobes, placed in close contact 

 along the middle line, situated partly in the anterior mediastinum, partly in the 

 neck, and extending from the fourth costal cartilage upwards, as high as the 

 lower border of the thyroid gland. It is covered by the sternum, and by the 

 origins of the Sterno-hyoid and Sterno-thyroid muscles. In the mediastinum, 

 it rests upon the pericardium, being separated from the arch of the aorta and 

 great vessels by the thoracic fascia. In the neck, it lies on the front and sides 

 of the trachea, behind the Sterno-hyoid and Sterno-thyroid muscles. The two 

 lobes generally differ in size ; they are occasionally united, so as to form a single 



