5 o8 TEXT-BOOK OF PHYSIOLOGY 



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 Schafer 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, nevertheless 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 generally. On the contrary, the splenic volume increase is coinci- 

 dent 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 vascu- 

 lar 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 

 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 possesses some 

 contractile mechanism independent to some extent of the nerve system. 



