THE THYMUS 131 



are imKluced in the s])leeii of the eiiil)ryo inammal, this ])()\ver is 

 later transferred to tlie red marrow of bones. lIowe\er, in certain 

 path()lo,u:ical conditions, erythroblasts and the three ty])es of myel- 

 ocytes can he found in the spleen, showin"; that in such cases 

 production of erythrocytes and fj;ranul()cytes can again occur here. 

 A recent review of lymphatics by 1 )rinker and Fields corroborates 

 an early account of Mollier concerning the ojjen nature of this 

 ])orti()ii of the vascular system — open to the extent that blood with 

 its cells can freely though sluggishly pass into diffuse lymphoid 

 tissue of the red pulp. The s])lenic sinusoids join com])lete \-enous 

 ca])illaries, which continue into small venules ])rojecting through 

 the red pulj) to enter the trabecular. In the s])leen, blood can leave 

 the vessels and mingle with surrounding cells without clotting. 

 Blo()d-])ressure is very low here, and ])eriodic })ulsations of the entire 

 spleen have been observed; a ])eriodic contraction forcing bloofi 

 from the ])ulp into the sinuses and veins is followed by a dilatation. 

 The smooth muscle in the capsule and trabecuhe effects this con- 

 traction. The connective tissue of the capsule limits the dilatation 

 or the accunuilation of too much blood as the contraction of the 

 smooth muscle imparts a gentle pressure forcing the blood l)ack 

 into the veins. The absence of cellular injury when blood enters 

 the red i)uli) from the sinusoids apparently accounts for the failure 

 of clotting of such extraxascular blood. 



THE THYMUS. 



The thymus occurs throughout the vertebrate groiips as a variable 

 structure beginning as invaginations of the ei)ithelium of the gill 

 clefts that in\-ade the underlying mesenchyme. The thyroid and 

 parathyroid anlages are similarly derived. In some fishes the 

 epithelial tissue appears predominant, but in most forms the asso- 

 ciated mesenchymal elements develop a lymphoid tissue about the 

 epithelial elements. This lymphoid tissue becomes more ])rominent 

 among reptiles, birds and mammals. The epithelial tissue is usually 

 cut off from the gill pouches during embryonic development. In 

 the lower forms the thymus may remain as a varying number of 

 small buds associated with the gill pouches from which they were 

 derived. In the mammals a conniion lol)ulated mass is usually 

 formed and lies along the lower part of the esojjhagus and extends 

 into the thorax over the pericardium. 



In general, a thin-walled capsule of fibroelastic connective tissue 



