SECRETION. 419 



tically uniform. The effect of the rhythmic contractions of the 

 splenic muscle tissue is to force the blood through the organ, a 

 condition necessitated perhaps by the pressure relations within, 

 though what function is thereby fulfilled is not apparent. 



It was subsequently shown by Schaf er and Moore that the splenic 

 volume is extremely responsive to all fluctuations of the arterial 

 blood-pressure; that though the spleen may passively expand and 

 recoil in response to the rise and fall of the blood-pressure, never- 

 theless the reverse conditions may obtain: viz., that the splenic 

 volume may diminish as the pressure rises, if the splenic arterioles 

 contract simultaneously with the contraction of the arterioles gener- 

 ally. On the contrary, the splenic volume may increase coincident 

 with a dilatation of the splenic and systemic arterioles^- In addition 

 to the rhythmic variations, the spleen steadily increases in volume 

 for a period of five hours after digestion, and then gradually returns 

 to its former condition.^^ 



Influence of the Nerve System. The nerves which supply the 

 vascular and visceral muscles in the spleen are derived directly 

 from the semilunar ganglion (post-ganglionic fibers) and pass to it 

 in company with the splenic artery. The nerve-cells from which 

 they arise are in physiologic relation with nerve-fibers (pre-ganglionic 

 fibers) which emerge from the spinal cord in the anterior roots of 

 the third thoracic to the first lumbar nerves inclusive, though they 

 are found most abundantly in the sixth, seventh, and eighth thoracic 

 nerves. Their center of origin is in the medulla oblongata. 



Stimulation of the nerves in any part of their course gives rise 

 to a diminution in splenic volume; division of the nerves is followed 

 by an increase in the volume. In asphyxia the spleen is small and 

 contracted, a condition Attributed to a stimulation of the centers 

 in the medulla by the venosity of the blood. 



The musculature of the spleen may also be excited to contraction 

 by reflex influences, as shown by the fact that stimulation of the 

 central end of a sensory nerve is attended by a diminution of volume. 



Inasmuch as the excised spleen will continue to exhibit variations 

 in volume when perfused with blood, it would appear that it possess 

 some mechanism independent to some extent of the nerve system. 



