THE BONE-MARROW. 2O/ 



from which they develop may lose their nuclei in the spleen pulp 

 or only after entering the circulation (compare Bone-marrow). 



Lymphatic vessels have been observed in the capsule and tra- 

 beculae, but not in the spleen pulp nor Malpighian corpuscles. 



The spleen receives medullated and nonmedullated nerve-fibers ; 

 the latter are much more numerous. The medullated nerve- 

 fibers are no doubt the dendrites of sensory neurones. Their 

 mode of ending has, however, not been determined. It is probable 

 that they will be found to terminate in the fibrous-tissue coat of the 

 vessels, and in the trabeculae and capsule. The nonmedullated 

 nerve-fibers, no doubt the neuraxes of sympathetic neurones, are 

 very numerous ; they enter the spleen with the artery and mainly 

 follow its branches. By means of the chrome-silver method, 

 Retzius (92) has shown that in the rabbit and mouse these nerve- 

 fibers follow the vessels, forming plexuses which surround them, 

 the terminal branches of these plexuses terminating in free endings 

 in the muscular coat of the arteries. Here and there a nerve-fiber 

 could be traced into the spleen pulp. The mode of ending of such 

 fibers could, however, not be determined. The nonstriated muscle- 

 cells of the trabeculae and capsule no doubt also receive their inner- 

 vation from the nonmedullated nerves (neuraxes of sympathetic 

 neurones). 



D. THE BONE-MARROW. 



The ingrowing periosteal bud which ushers in the process of 

 endochondral ossification constitutes the first trace of an embryonal 

 bone-marrow (compare p. 117). It consists mainly of elements 

 from the periosteum which penetrate with the vascular bud and later 

 form the entire adult bone-marrow. The red bone-marrow is formed 

 first. This is present in embryos and young animals, and is devel- 

 oped from the above elements during the process of ossification. 

 As Neumann (82) has shown, the red bone-marrow of the human 

 embryo is first formed in the bones of the extremities and gradually 

 replaced in a proximal direction, so that in the adult it is found 

 only in the proximal epiphyses, in the flat bones and in the 

 bodies of the vertebrae. In the remaining bones and parts of bones 

 the red bone-marrow is replaced by the yellow bone-marrow (fat- 

 marrow). 



As a result of hunger and certain pathologic conditions the yel- 

 low bone-marrow changes into a gelatinous substance, which, how- 

 ever, may again assume its original character. 



The red bone-marrow, surrounded by a delicate fibrous-tissue 

 membrane, the endosteum, is a tissue consisting of various cellu- 

 lar elements imbedded in a matrix of reticular tissue, which has 

 been demonstrated by Enderlen with the chrome-silver method, 

 and which is similar to the adenoid reticulum. Aside from these 

 cellular elements, the marrow contains numerous vessels (see below), 

 fixed connective-tissue cells, etc. 



