LYMPHATIC ORGANS 101 



demarcation between the two layers becomes almost entirely lost. Recent 

 investigations have made it seem probable that the spleen develops in part at 

 least from these mesothelial cells which grow down into the mesenchyme. In 

 this case it is probable that mesenchyme gives rise to capsule, trabeculte, and 

 reticular connective tissue — the connective-tissue framework of the organ — 

 while mesothelium is responsible for some at least of the various cellular 

 elements of the splenic pulp. 



Lymphatics arc not numerous. In certain of the lower animals 

 large lymph vessels occur in the capsule and septa. These are not 

 well developed in man. Lymph vessels are present in the connective 

 tissue of the hilum. They probably do not occur in the splenic pulp 

 or in the splenic corpuscles. 



Nerves. — These arc mainly non-medullated, although a few 

 medullated fibres are present. Among the latter are dendrites of 

 sensory neurones whose cell bodies arc situated in the spinal gan- 

 glia. They supply the connective tissue of the capsule, septa, and 

 blood-vessels. The non-medullated fibres— axones of sympathetic 

 neurones — accompany the arteries, around which they form plexuses. 

 From these plexuses terminals pass to the muscle cells of the arteries, 

 to the septa, to the capsule, and possibly also to the splenic pulp. 

 The exact manner in which both medullated and non-medullated 

 fibres terminate is as yet undetermined. 



TECHNIC 



(i) The spleen of a cat is more satisfactory for topography than the human 

 spleen, as it is smaller, contains more connective tissue and its Malpighian bodies 

 are more evenly distributed and more circumscribed. Fix in formalin-Miiller's 

 fluid (technic 6, p. 7), and harden in alcohol. Cut sections through the entire 

 spleen. Stain with ha;matoxylin-eosin (technic i, p. 20), or with haematoxylin- 

 picro-acid-fuchsin (technic 3, p. 21). 



(2) Human Spleen. — Small pieces are treated as in technic (i). 



(3) Human Spleen (Congested). — Congested human spleens are usually 

 easy to obtain from autopsies. Treat as in technic (i). The cavernous veins 

 being distended with blood, the relations of the veins to the pulp cords are more 

 easily seen than in the uncongested spleen. The contrasts are especially sharp 

 in sections stained with haimatoxylin-picro-acid-fuchsin. 



(4) The cells of the spleen may be studied along the torn edges or in the thin- 

 ner parts of any of the spleen sections. Or a smear may be made in a man- 

 ner similar to that described in technic (page 114), by drawing the end of a 

 slide across a freshly cut spleen surface and then smearing the tissue thus ob- 

 tained across the surface of a second slide. Dry, fix in equal parts alcohol and 

 ether (one-half hour), stain with haematoxylin-eosin and mount in balsam. Or 

 the cut surface of the spleen may be scraped with a knife, the scrapings trans- 



