THE HEART. 



689 



Megakar>ocyle 



body, has been challenged by Hammar and by Maximow,^ who found lymphocytes 

 in the blood and connective tissues before they appear within the thymus. It is 

 probable that the early lymphocytes also originate from mesenchymal cells outside 

 the vessels, which they later enter, aided by their migratory powers. Their subsequent 

 multiplication is effected by division, for the most part mitotic, of the pre-existing 

 cells. This proliferation occurs chiefly within the lymphoid tissue throughout the 

 body, the lymph-nodules, spleen and bone-marrow being the most important local- 

 ities. The germ-centres of the lymph-nodules (page 936) are seats of especial 

 activity for the formation of the types of colorless cell known as the mononuclear 

 lymphocyte, although whether the proliferating cells originate within the germ- 

 centres, or only complete their division in these situations after being carried from 

 other points (Stohr), is still unsettled. 



From the developmental standpoint, the sharp separation of the colorless blood- 

 cells into lymphocytes and leucocytes, as insisted upon by Ehrlich and his supporters, 

 based on the assumption that the leucocytes originate exclusively within bone-marrow, 

 is not well founded in view of the presence of all the typical forms of white cells, in- 

 cluding the polymorphonuclear leucocytes, shortly after the first appearance of the 

 white corpuscles and long before 



the advent of the earliest bone- Fig. 653. 



marrow (Ebner). For the pres- 

 ent, at least, it seems most reason- 

 able to regard the various forms 

 of the white cells as constituting 

 a genetic sequence in which the 

 lymphocyte, leucocyte, and eosin- 

 ophil represent different stages in 

 the development of elements hav- 

 ing a common origin. 



In addition to the red blood- 

 cells in various stages of develop- 

 ment and the different types of 

 leucocytes, peculiar huge elements 

 early appear in the embryonic 

 blood-forming organs, and after 

 birth in bone-marrow. These 

 giant cells, or 7negakaryocytes 

 (Howell), are distinguished by 

 their large, irregularly lobulated 

 but single nucleus from the osteo- 

 clasts, since the nuclei of the latter are usually oval and multiple. The megakaryo- 

 cytes are often observed containing within their substance the remains of both white 

 and red cells ; they are, therefore, regarded as phagocytes upon which devolves the 

 removal of effete blood-corpuscles. Their origin is uncertain, by some (Howell, van 

 der Stricht, Heidenhain) being referred to the leucocytes, and by others (Kolliker, 

 Kuborn) to the endothelium of the vessels, while Ebner regards those within the bone- 

 marrow as probably derived from fixed connective-tissue cells of the reticulum. Nei- 

 ther form of these giant marrow-cells is normally found within the post-natal circulation. 



Nucleated 

 red cells 



Leucocytes 



Osteoclast 



Bone V 



trabecula 



Osteoblasts 



Section of embryonal bone-marrow, showing 

 nucleated erythrocytes, leucocytes and mega- 

 karyocyte. '/. 625. 



. THE HEART. 



General Description. — The heart is a hollow, muscular organ of a somewhat 

 conical shape, situated in the lower part of the thoracic cavity, behind the lower two- 

 thirds of the sternum. It is enclosed within a double-walled serous sac, the pericar- 

 dhcm, and has a somewhat oblique position in the thorax, its base (basis cordis) looking 

 upward, dorsally, and to the right, while its apex (apex cordis) points downward, ven- 

 trally, and to the left. In consequence of this obliquity about two-thirds of the organ 

 lies to the left and one-third to the right of the median plane of the body. 



^ Archiv f. mikros. Anatom., Bd. Ixxxiv., 1909. 



44 



