THE THORAX 1019 



presents a beaded appearance, with constrictions between the projections, these 

 constrictions corresponding to the attached margins of the valvular segments. 

 Development. — According to Sabin the lymphatic vessels are developed 

 from the endothelial lining of the primitive lymph-sacs (see Development of 

 Lymphatic System). 



Strneture of Lymphatic Glands. 



A lymphatic gland consists of a capsule, which encloses the glandular 

 substance. The capsule is composed of connective tissue, containing elastic 

 fibres. 



The glandular substance presents two parts — superficial or cortical, and 

 central or medullary. Each of these parts is permeated by a supporting 

 framework of trabecula; derived from the capsule. These trabecula? are 

 composed of connective tissue, with a few plain muscular fibres. They sub- 

 divide the cortex into follicles, between which they form incomplete septa. 

 In the medulla the trabeculae are arranged in a reticular manner. 



The glandular substance is formed by lymphoid or adenoid tissue, which 

 consists of retiform tissue, with lymphocytes in its meshes. In the cortex 

 of the gland this lymphoid tissue is disposed as lymph-follicles, and in the 

 medulla it forms lymph-cords. In both parts of the gland there are spaces 

 between the glandular substance and the supporting trabecular framework. 

 These spaces, which are for the passage of lymph, constitute the lymph-sinus. 

 This sinus is broken up at all parts of the gland by retiform tissue, the meshes 

 of which are partially lined with branched cells of the nature of phagocytes. 

 The glandular substance, except the lymph-sinus, are permeated by blood- 

 capillaries. 



Lymphatic glands are furnished with bloodvessels, nerves, and lymphatic 

 vessels. The arterioles pass to the glandular substance, being at first en- 

 sheathed by the supporting trabeculae of connective tissue, and subsequently 

 by the retiform tissue of the glandular substance. The nerves are destined 

 for the plain muscular tissue of the trabeculae and bloodvessels. 



The lymphatic vessels are of two kinds — afferent and efferent. The afferent 

 vessels enter the gland over its surface, whilst the efferent vessels emerge at a 

 definite part of the gland, where there is a shght depression, called the hilum. 

 The afferent vessels convey lymph to the cortical part of the lymph-sinus, 

 whilst the efferent vessels convey lymph from the medullary part of that 

 sinus. Inasmuch as the medullary part of the gland extends quite to the 

 surface at the place where the depression, known as the hilum, exists, the 

 efferent vessels emerge from the gland through this hilum, which also gives 

 passage to the arteries and veins. 



Development. — Lymphatic glands are developed partly from the primitive 

 lymph-sacs, and partly from peripheral lymphatic vessels (see Development 

 of Lymphatic System). 



Trachea in the Thorax. — ^The thoracic portion of the trachea 

 extends from the level of the upper border of the manubrium sterni 

 to the level of the intervertebral disc between the bodies of the 

 fourth and fifth thoracic vertebrae, where it divides into the two 

 bronchi, right and left. It occupies a median position in the 

 superior mediastinum, and its length is about 2J inches. As in 

 the neck, it is cylindrical and firm in front and at the sides, but 

 posteriorly it is flattened and membranous, where it rests upon the 

 oesophagus. 



Relations — Anterior. — ^The manubrium sterni, and the origins of 

 the sterno-hyoid and sterno-thyroid muscles ; the remains of the 

 thymus body ; the origins of the innominate and left common 

 carotid arteries, and the left innominate vein; and the arch of the 

 aorta, which lies upon it immediately above its bifurcation into the 



