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1312 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



line masses of spleen-pulp about 1 mm. in diameter, known as splenic lobules. Each lobule is 

 bounded by three main trabeculse, from each of which secondary trabeculaj pass into the sub- 

 stance of the lobule incompletely subdividmg it into compartments, filled with splenic pulp, 

 arranged in the form of anastomosing columns or cords and designated as pulp-cords. The 

 branches of the splenic artery, after coursing for a short distance in the main trabeculie, leave 

 these, and, after further division, become surrounded with a layer of adenoid tissue, which layer 

 presents here and there irregular thickenings forming the Malpighian corpuscles. An arterial 

 branch, surrounded with adenoid tissue, enters the apex of a splenic lobule, constituting its 

 intralobular vessel, which, soon after entering the lobule, loses its adenoid sheath and then 

 sends a branch to each of the above-mentioned compartments. These branches do not anas- 

 tomose. They give off terminal branches which course in the pulp-cords, form dilations, 

 ampullag, and terminate directly or indirectly in the large venous spaces found between the pulp- 

 cords. From the latter the blood passes, by means of small intralobular veins, to interlobular 

 veins situated in the trabeculis boimding the lobules. Some of the ampuUse are connected with 

 one another by capillary branches. 



Blood-supply. — The spleen receives its blood from the splenic artery, which is very large in 

 proportion to the size of the organ it supplies. It divides in the phrenicolienal ligament into 

 from three to six or eight branches, rami lienales (fig. 1062), which enter the spleen at the hilus. 

 After entering the spleen the arteries divide and subdivide and run to their termination in the 

 ampullae without anastomosing. They form what are known as terminal arteries. The main 

 splenic artery is very tortuous. The vein, vena lienalis, leaves the spleen usually by the same 

 number of branches as the entering artery. These unite in the phrenicolienal ligament to form 

 a large trunk which is straighter than the splenic artery and lies caudal to it. 



The lymphatics. — A superficial and a deep set of lymphatics have been described in the 

 spleen. The former is said to form a plexus beneath the peritoneum and the latter to be derived 

 from the fine perivascular spaces in the adenoid tissue aroimd the vessels. From these several 

 trunks arise and joining at the hilus pass between the layers of the phrenicolienal ligament to 

 empty into the lymph-glands dorsal to and around the cephalic border of the tail of the pancreas. 

 The presence of both superficial and deep sets of lymphatics in the human spleen has been denied 

 by some investigators. According to Mall, there is no deep set. 



The nerves. — The nerves are derived from the right vagus and from the coeliac plexus. 

 They enter the spleen at the hilus, accompanying the branches of the lienal artery. They are 

 ■composed mostly of non-medullated fibres which form a rich plexus around the arteries supply- 

 ing the muscular fibres in the media while a second group has been traced to the muscular fibres 

 of the trabeculse. 



Development of the spleen. — The first anlage of the spleen is seen in the fifth week of foetal 

 life as a swelling on the dorsal (left) surface of the mesogastrium. This is due to an increase in 

 the mesenchymal cells as well as to a thickening of the coelomic epithelium. This latter becomes 

 stratified, and indistinctly differentiated from the underlying embryonic connective tissue 

 through the transformation of the deepest of the epithelial cells into mesenchymal cells. As 

 development proceeds the thickened mass becomes entirely isolated and the coelomic epithelium 

 covers it as a single layer. 



The arteries are seen first as a capillary network throughout the organ which considerably 

 later become arranged as tufts of widened capillaries, the anlage, of the vascular structural unit. 

 These spherical groups of arterial capillaries leading by wide openings into a wide meshed venous 

 plexus are boimded by trabeculse from the capsule. The number of structural units in the 

 spleen seems to be fixed fairly early but the size and complexity changes greatly. The spherical 

 mass with a single central artery changes to the adult condition where the central artery gives 

 off side branches, each of which has a spherical mass of capillaries, and the pulp intervenes 

 between the artery and the vein so that the capillary circulation of the early embryo becomes 

 the cavernous circulation of the adult. The lienal lymphatic nodules of Malpighi and the splenic 

 pulp appear only in the latter half of embryonic life. 



THYREOID GLAND . 



The thyreoid gland [glanduhi thyreoidea] is an extremely vascular, ductless 

 gland, whose internal secretion, acting as a stimulus to the tissues, has a profound 

 influence on the nutrition of the body and on the nervous system. It is a single 

 organ composed of two lateral, frequently unsymmetrical, masses, joined to- 

 gether by a transverse median band. The median transverse band or isthmus 

 [isthmus gl. thyreoidcffi] is thin and narrow, and often has a long slender process, 

 the pyramidal lobe [lobus pyramidalis], extending from it cephalically. The lat- 

 eral parts or lobes [lobi, dexter et sinister] form the ])rinoipal mass of the gland. 



It is situated in tlie ventral portion of the middle third of the neck on both 

 sides of the larynx and the cephalic end of the trachea, dorsal to the infrahyoid 

 group of muscles. 



The consistency of the thyreoid gland is uniformly soft and compressible. The 

 colour is reddish, with a brownish or yellowish cast, but becoming more bluish or 

 reddish with changes in its blood content. 



The size is subject to considerable individual variation and is slightly greater 

 in women than in men. The normal thyreoid gland measures from 4 to G cm. 

 in width at its widest part. The lateral lobes measure from 5 to 8 cm. in length, 



