THE THORAX 945 



The veins pass to the internal mammary, superior phrenic, and 

 azygos veins. 



Nerve-supply. — The phrenic, pneimiogastric, and sympathetic 

 nerves. 



Lymphatics. — ^These pass to the anterior, superior, and posterior 

 mediastinal glands. 



Structure. — The fibrous portion of the pericardium is composed of fibrous 

 tissue, and is very dense, but not very extensible. The serous portion 

 consists of a homogeneous, connective-tissue basement membrane, con- 

 taining some elastic fibres, and lined with endothelium. The parietal layer 

 is much thicker than the visceral, and the latter is intimately connected 

 with the cardiac muscular tissue, except along the grooves which are occu- 

 pied by adipose tissue and bloodvessels. 



Development. — The serous portion of the pericardium is developed from 

 the walls of the coelom, or body-cavity, which is the cleft in the mesoblast 

 separating the splanchnopleure and somatopleure. 



Thymus Body (Gland). — ^This body is present only in the foetus 

 and young child. It attains its greatest size at the period of 

 birth, and forms a very conspicuous object in the dissection of 

 a child during the first year or two of life. It soon, however, 

 begins to diminish. In its fully - developed condition it is 

 situated partly in the thorax, and partly in the neck. In the 

 former situation it occupies the superior and anterior media- 

 stinal spaces, extending as low as about the level of the fourth 

 costal cartilages, and lying in front of the great bloodvessels and 

 upper part of the pericardium, the upper part of the sternum 

 being in front of it. In the neck it extends as high as the lower 

 part of the thyroid body, being imder cover of the sterno-hyoid 

 and sterno-thyroid muscles. In this situation it embraces the 

 front and sides of the trachea, completely concealing it from view, 

 and it encroaches upon the carotid sheath on either side. Its 

 length is about 2j inches, and its breadth, which is greatest in- 

 teriorly, is about i^ inches. Its colour is pinkish ; it is soft in con- 

 sistence ; and its surface shows indications of lobulation. It consists 

 of two asymmetrical lateral lobes, each of which is pyramidal. In 

 some cases the left lobe is the larger, and in other cases the right. 

 These lobes are in close contact, but are still quite distinct. Some- 

 times a third lobe is present, occupying an intermediate position 

 between the other two. 



Blood-supply. — ^The arteries are chiefly derived from the internal 

 mammar}^ inferior thyroid, and superior thyroid. 



The veins open into the right and left innominate and thyroid veins. 



Nerve-supply, — The nerves are derived from the sympathetic 

 and pneumogastric. 



Structure.— Each lobe has a capsule of fibrous tissue, from which trabe- 

 culae pass into the interior. These trabeculae map out each lobe into large and 

 small lobules. Each lobule consists of an outer cortical, and an inner medullary, 

 portion. The cortex is composed of lymphoid tissue, the lymphoid element 

 predominating over the retiform, and it is surrounded by a capillary net- 

 work of bloodvessels, which contains many lymph corpuscles. The lymphoid 

 tissue of the cortex is incompletely subdivided into nodules by means of 

 trabeculae. 



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