THE DUCTLESS GLANDS. 



rMHERE are certain organs, in various situations, which are very similar to 



J_ secreting gland?, but differ from them in one essential feature viz., they do 



not possess any ducts by which their secretion is discharged. These organs 



are known as the ductless glands. They are capable of internal secretion that 



is to say, of forming, from materials brought to them by the blood, substances 



which have a certain influence upon the nutritive and other changes going on 



in the body. This secretion is carried into the blood stream, either directly by 



the veins or indirectly through the medium of the lymphatics. 



These glands include the thyroid and the parathyroids, the thy my s, the spleen, 

 the suprarenal glands, and me small carotid, coccygeal , ancT parasympa thetic 

 bodies, which will be described in this section. They also include the lymph 

 nodes (or glands) which have already been described on pages 771 to 805; and 

 the epjphysis (pineal gland) and hypophysis (pituitary) described with the brain 

 on pages 907 and 910. Certain isolated cell masses in the pancreas and the testicle, 

 apparently engaged in internal secretion, are described with those organs. 



THE THYROID GLAND OE BODY (GLANDULA THYREOIDEA) 



(Fig. 1194). 



The thyroid gland is a highly vascular organ, situated at the front and sides of 

 the neck, and extending upward upon each side of the larynx; it consists of two 

 lateral lobes connected across the middle line by a narrow transverse portion, 

 the isthmus. 



The weight of the gland is somewhat variable, but is usually about one ounce. 

 It is somewhat heavier in the female, in whom it becomes enlarged during men- 

 struation and pregnancy. 



The lateral lobes are conical in shape, the apex of each being directed upward 

 and outward as far as the junction of the middle with the lower third of the thy- 

 roid cartilage; the base looks downward, and is on a level with the fifth or sixth 

 tracheal ring. Each lobe is about two inches (5 cm.) in length, its greatest width 

 is about one inch and a quarter (3 cm.), and its thickness about three-quarters 

 (2 cm.) of an inch. The summit of the lateral lobe not unusually is pointed and 

 reaches to the level of the oblique line upon the ala of the thyroid cartilage or even 

 higher. The right lobe is, as a rule, somewhat larger than the left. The lower 

 portion of the gland, when the head is extended, is about one inch above the 

 upper margin of the sternum; when the head is flexed, it is at the level of the 

 upper border of the sternum or even below and behind it. 



The external or superficial surface is convex, and covered by the skin, the super- 

 ficial fascia, the deep fascia, the Sternomastoid, the anterior belly of the Omo- 

 hyoid, the Sternohyoid, and Sternothyroid muscles, and beneath the last-named 

 muscles by the pretracheal layer of the deep fascia, which forms a capsule for 

 the gland (Fig. 299). 



The deep or internal surface is moulded over the underlying structures viz., 

 the thyroid and cricoid cartilages, the trachea, the Inferior constrictor and pos- 



( 1437 ) 



