946 A MANUAL OF ANATOMY 



The medulla is more transparent than the cortex, the retiform element of 

 the lymphoid tissue is more conspicuous, and the lymph corpuscles are less 

 numerous. In addition, the medulla contains small groups of cells, more or 

 less concentrically arranged, which are known as the concentric corpuscles 

 of Hassall. According to one view these cells are remains of the diverticula 

 from which the thymus body is developed ; but another view is that they 

 are concerned with the formation of bloodvessels and connective tissue. 



Development. — The thymus body is developed in two lateral parts from 

 the entodermic epithelium of the pharyngeal portion of the primitive gut. 

 The epithelium of the third visceral cleft, on either side, becomes evaginated, 

 and gives rise to a hollow diverticulum. This diverticulum has a thick 

 epithelial wall, but a small lumen, and it grows in a downward direction. 

 Its distal end, which lies ventral to the pericardium, forms a solid enlarge- 

 ment, and the proximal end loses its connection with the third visceral cleft. 

 The enlarged distal end gives off numerous solid epithelial buds, which are 

 invested with mesoderm. This budding gradually extends to the proximal 

 part of the diverticulum. The solid buds ramify freely and give rise to the 

 corresponding lobe of the thymus. The original diverticulum thus assumes 

 a lobulated appearance, and resembles a racemose gland. The buds or acini, 

 however, are soUd, and not hollow, as in racemose or acinous glands. The 

 acini are separated by connective tissue and bloodvessels, which are developed 

 from the surrounding mesoderm. Lymphoid tissue is also developed around 

 the acini, and this tissue forms the greater part of the adult thymus. The 

 epithelial elements of each lobe are subordinate to the lymphoid tissue, and 

 are ultimately represented by the concentric corpuscles of Hassall. 



Thymus Body in the Adult. — Only the remains of the thymus body 

 are present in the form of a collection of adipose and areolar 

 tissues, which lies in the superior mediastinum in front of the 

 innominate and left common carotid arteries, above the arch of 

 the aorta. This mass receives a few twigs from the mediastinal 

 branches of the internal mammary artery, and it usually contains 

 a certain amount of the original lymphoid tissue. 



Lungs. — The lungs are two in number — right and left. They are 

 spongy in consistence, float in water, and are readily compressed. 

 When pressed between the fingers crepitation is elicited, this being 

 due to the displacement of air. When the lung is incised, similar 

 crepitation is heard, and a muco-serous fluid, mixed with air, 

 exudes. They possess considerable elasticity, their colour is that 

 of a dark slate, and they are usually mottled, this being due to 

 carbonaceous matter. In early life, however, the colour is rose- 

 pink. 



The lungs occupy the greater part of the thoracic cavity. Nor- 

 mally they are at all times in close contact with the thoracic walls, 

 the pleurae intervening. Unless adhesions have formed during 

 life between the visceral and parietal pleurae, the surface of each 

 lung is quite free except in two situations, namely, at the root, 

 which occupies a limited area of the inner surface, and at the 

 attachment of the ligamentum latum. Each lung is conical, the 

 base being directed downwards. It presents for consideration an 

 apex, a base, two surfaces, outer and inner, and two borders, anterior 

 and posterior. 



The apex is blunt, and rises out of the thoracic cavity into the 



