BRONCHI — BRONCHIAL TUBES. 335 



anterior and middle parts, consist of twigs from the carotid ; 

 posteriorly the supply comes from the broncho-cesophageal ; th& 

 nerves are derived from the recurrent and sympathetic. 



THYROID AND THYMUS GLANDS. 



About the two first tracheal rings is the brownish-red, thyroid 

 body, consisting of two lobes, right and left, joined by a narrow 

 band, the isthmus. Each lobe is ovoid, and consists of minute 

 vesicles surrounded by a plexus of capillaries, and connected by 

 areolar tissue. The gland is ductless, but copiously supplied 

 with blood by branches of the carotid, and secretes an albumin- 

 ous fluid, which is perhaps absorbed by the lymphatics, and 

 conveyed into the blood. It is very large in foetal life, but its 

 use is unknown. 



The thymus is another ductless gland, situated on the inferior 

 aspect of the trachea, and above the sternum. It consists of 

 two halves united by areolar tissue, and is composed of lobules. 

 disposed round a central canal, having no external openino". 

 Within are vesicles similar to those of the thyroid body ; in the 

 foetus it is attached to the thyroid, and is large iat birth, 

 gradually disappearing. 



BRONCHI — BRONCHIAL TUBES. 



The terminal branches of the trachea are the right and left 

 bronchi, which enter the lungs and subdivide dichotomously inta 

 branches termed bronchial tubes, the latter becoming gradually 

 smaller, and finally terminating in the air-cells. The entire 

 ramification when isolated has the appearance of a tree, the 

 trachea being the main trunk, the bronchi and bronchial tubes 

 the branches, and the air-cells the leaves. These structures are 

 accompanied throughout by the arteries, veins, and nerves, which 

 Lave the same arborescent distribution. The right bronchus is 

 the larger, the left the longer, since it passes under the aorta before 

 reaching the lung. 



The bronchi and bronchial tubes are made up of cartilaginous 

 rings, differing only from those of the trachea in being made up 

 of several pieces, which overlap and are united by cellular tissue 

 on their inner surface. As the tubes diminish in size, the num- 

 ber of these pieces is diminished, and ultimately they disappear. 



