THYROID GLAND. 787 



other, and do not anastomose ; and, according to Dr. "Waters, those of the sepa- 

 rate groups of intercellular passages, or air-sacs (which groups he denominates 

 lobulettes), are also independent ; so that in the septa between two adjoining 

 lobulettes, there would be a double layer of capillaries, one layer belonging to 

 each of the adjacent air-sacs, or intercellular passages. If this is really the 

 arrangement of the vessels, it would follow, that in the septa between, the air- 

 cells (or alveoli), the blood in the capillaries would be exposed on all sides to 

 the action of the air, since it is circulating in a single layer of vessels, which is 

 in contact with the membrane of the air-passages on both sides ; but that, in 

 the septa between the intercellular passages (or air-sacs) the blood in the double 

 layer of capillaries will be in contact with the air on one side only. 



The Bronchial Arteries supply blood for the nutrition of the lung : they are 

 derived from the thoracic aorta, and, accompanying the bronchial tubes, are 

 distributed to the bronchial glands, and upon the walls of the larger bronchial 

 tubes and pulmonary vessels, and terminate in the deep bronchial veins. Others 

 are distributed in the interlobular areolar tissue, and terminate partly in the 

 deep, partly in the superficial, bronchial veins. Lastly, some ramify upon the 

 walls of the smallest bronchial tubes, and terminate in the pulmonary veins. 



The Superficial and Deep Bronchial Veins unite at the root of the lung, and 

 terminate on the right side in the vena azygos; on the left side, in the superior 

 intercostal vein. 



According to Dr. "Waters, the bronchial veins do not exist within the proper 

 substance of the lung, but commence at or near the root of the lung, by branches 

 which lie on the large bronchial tubes. He also denies that the bronchial arte- 

 ries contribute to the formation of the pulmonary plexus, believing that the 

 communication between the bronchial and pulmonary system of vessels takes 

 place in the pulmonary veins. If this view be correct, almost the whole of the 

 blood carried by the bronchial arteries must be returned to the heart by the 

 pulmonary veins, and thus the great mass of pure, or arterial blood which is 

 carried by the .pulmonary veins, would be adulterated by a small quantity of 

 carbonized or venous blood, which has passed through the bronchial circulation. 



The Lymphatics consist of a superficial and deep set : they terminate at the 

 Toot of the lung, in the bronchial glands. 



Nerves. The lungs are supplied from the anterior and posterior pulmonary 

 plexuses, formed chiefly by branches from the sympathetic and pneumogastric. 

 The filaments from these plexuses accompany the bronchial tubes, upon which 

 they are lost. Small ganglia are found upon these nerves. 



THYROID GLAND. 



The Thyroid Gland bears much resemblance in structure to other glandular 

 organs, and is usually classified together with the thymus, suprarenal capsules, 

 and spleen, under the head of ductless glands, since it has no excretory duct. Its 

 function is unknown, but, from its situation in connection with the trachea and 

 larynx, the thyroid body is usually described with those organs, although it 

 takes no part in the function of respiration. It is situated at the upper part of 

 the trachea, and consists of two lateral lobes, placed one on each side of that 

 tube, and connected together by a narrow transverse portion, the isthmus. 



Its anterior surface is convex, and covered by the Sterno-hyoid, Sterno-thy- 

 roid, and Omo-hyoid muscles. 



Its lateral surfaces, also convex, lie in contact with the sheath of the common 

 carotid artery. 



Its posterior surface is concave, and embraces the trachea and larynx. The 

 posterior borders of the gland extend as far back as the lower part of the 

 pharynx. 



The thyroid is of a brownish-red color. Its weight varies from one to two 

 ounces. It is larger in females than in males, and becomes slightly increased in 



