THE THYROID GLAND 457 



a continuous layer of unstriped muscle, comjiosed of circularl}^ arranged bundles. 

 The mucous membrane i)rcsents numerous longitudinal folds; it contains many 

 elastic fibers, mucous glands, and lymph nodules, and is lined by ciliated columnar 

 epithelium. As the tubes diminish in size the coats become thinner and the car- 

 tilages smaller; in tubes about 1 mm. in diameter the cartilages and mucous glands 

 are absent. 



By repeated branching the interlobular bronchi are formed, and from these 

 arise the lobular bronchioles. The latter enter a lobule and branch within it, 

 forming the respiratory bronchioles ; these give off the alveolar ducts, the walls of 

 which are pouched out to form hemispherical diverticula, the alveoli or air-cells. 



A lobule of the lung, the unit of lung structure, is made up of a lobular Ijronchi- 

 ole with its i)ranches, and their air-cells, l)lood- and lym]ih-vessels, and nerves. 

 Between the lobules is the interlobular tissue, which forms the supporting frame- 

 work of the lung. 



Vessels of the Lungs. — The branches of the pulmonary artery carry venous 

 blood to the lungs. They accompany the bronchi, and form rich capillary plexuses 

 on the walls of the alveoli. Here the blood is arterialized, and is returned to the 

 heart by the pulmonary veins. The bronchial arteries are relatively small vessels 

 which carry arterial l)lood for the nutrition of the lungs. The branches of these 

 arteries accompany the l)ronchial ramifications as far as the alveolar ducts, but 

 do not extend to the alveoli. The oesophageal artery also supplies pulmonary 

 branches (in the horse) which reach the lung by way of the ligament of the 

 latter. These branches vary much in size, and ramify chiefly in the subpleural 

 tissue of the basal part of the lung.' Th(^ lymph vessels are numerous, and are 

 arranged in two sets. The superficial set forms close networks in and under the 

 pleura, while the deep set accompanies the bronchi and pulmonary vessels. All 

 converge to the root of the lung and enter the bronchial lymph glands. 



Nerve-supply. — The pulmonary nerves come from the vagus and sympathetic 

 nerves. They enter at the hilus and supply branches to the bronchial arteries and 

 the air-tubes. 



THE THYROID GLAND 



The thyroid gland (CJlandula thyreoidea) is a very vascular ductless gland, 

 situated on the trachea close to the larynx. It is red brown in color, and consists 

 of two lateral lobes and a connecting isthmus (Fig. 354). 



The lateral lobes are situated on either side of the first and second or second 

 and third rings of the trachea, to which they are loosely attached. Each is al)out 

 the size of a walnut, and has a convex superficial face which is related to the parotid 

 gland and the omo-hyoid muscle, and a slightly concave, deep face, applied to the 

 trachea. 



The isthmus in the adult horse is usually very rudimentary. It may occur as a 

 very narrow glandular band which connects the posterior extremities of the lateral 

 lobes, extending across the ventral surface of the trachea, but it is frequently only 

 a small strand of connective tissue, and sometimes is entirely absent. It may be 

 represented by a small tail-like process of one lobe. 



The lateral lobes vary in size and position, and are often dissimilar on the two sides. The 

 anterior extremity is the larger; it is rounded and usually lies about a finger's breadth behind 

 the larynx, but may be in contact witli it. The posterior extremity is often produced to form a 

 narrow tail-like process, which is continuous with the isthmus when one is present. In the ass 

 and mule the isthmus is usually well developed. There is sometimes a narrow band of thyroid 



1 It must not be inferred from the necessarily brief account here given that the two sets of 

 vessels are quite distinct. On the contrary, competent observers state that numerous small 

 branches of the bronchial arteries anastomose with pulmonary vessels. Part of the blood con- 

 veyed to the lungs by the bronchial arteries is returned by the pulmonary veins. In the horse 

 bronchial veins are absent or inconstant. 



