722 ORGANS OF VOICE AND RESPIRATION. 



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 gela- 

 tin, stearin, olein, extractive matter, alkaline and earthy salts, and water. The 

 salts are chloride of sodium, alkaline sulphate, phosphate of potash, lime, mag- 

 nesia, and a trace of oxide of iron. 



THYMTJS GLAND. 



The Thymus Gland presents much resemblance in structure to other glandular 

 organs, and is classified under the head of the ductless glands, from its possessing 

 no excretory duct. 



The thymus 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 mass, and sometimes 

 separated by an intermediate lobe. The thymus is of a pinkish-gray color, soft, 

 and lobulated on its surfaces. It is about two inches in length, one and a half in 

 breadth, below, and about three or four lines in thickness. At birth, it weighs 

 about half an ounce. 



Structure. Each lateral lobe is composed of numerous lobules, held together by 

 delicate areolar tissue, the entire gland being inclosed in an investing capsule of 

 a similar, but denser, structure. The primary lobules vary in size from a pin's 

 head to a small pea. Each lobule contains, in its interior, a small cavity, which 

 is surrounded with smaller or secondary lobules, which are also hollow within. 

 The cavities of the secondary and primary lobules communicate ; those of the 

 latter opening into the great central cavity or reservoir of the thymus, which 

 extends through the entire length of each lateral half of the gland. The central 

 cavity is lined by a vascular membrane, which is prolonged into all the subordinate 

 cavities, and contains a milk-white fluid resembling chyle. 



If the investing capsule and vessels, as well as the areolar tissue connecting 

 the lobules, are removed from the surface of either lateral lobe, it will be seen 

 that the 'central cavity is folded upon itself, and admits of being drawn out into a 

 lengthened tubular cord, around which the primary lobules are attached in a 

 spiral manner, like knots upon a rope. Such is the condition of the organ at an 

 early period of its development ; for Mr. Simon has shown, that the primitive 

 form of the thymus is a linear tube, from which, as its development proceeds, 

 lateral diverticula lead outwards, the tube ultimately becoming obscure, from its 

 surface being covered with numerous lobules. 



According to Oesterlen and Mr. Simon, the cavities in the secondary lobules 

 are surrounded by rounded saccular dilatations or vesicles, which open into it. 

 These vesicles are formed of a homogeneous membrane, inclosed in a dense capil- 

 lary plexus. 



The whitish fluid contained in the vesicles and central cavity of the thymue 



