4 68 TEXT-BOOK OF PHYSIOLOGY. 



utes to the formation of blood. The presence of disintegrated red blood- 

 corpuscles has suggested the view that the spleen exerts a destructive action 

 on functionally useless red corpuscles. These and other theories as to 

 splenic functions have been offered by different observers, but all are lack- 

 ing positive confirmation. 



Volume Variations of the Spleen. It was shown some years since by 

 Roy, with the aid of the plethysmograph, that the spleen undergoes rhyth- 

 mic variations in volume from moment to moment. In the cat and in the 

 dog the diminution in the volume (the systole) and the increase in volume 

 (the diastole) together occupied about one minute. 



This fact was determined by withdrawing the spleen through an opening 

 in the abdominal wall and enclosing it in a box with rigid walls, the interior 

 of which was connected with a piston recording apparatus. The system 

 being filled with oil, each variation in volume was attended by a to-and-fro 

 displacement and a corresponding movement of the recording lever. The 

 special form of plethysmograph used for this purpose is known as the on- 

 cometer or bulk measurer, and the recording apparatus as the oncograph. 



The cause of these variations in volume Roy attributed to a rhythmic 

 contractility of the non-striated muscle-fibres in the capsule and trabeculae, 

 and not to changes in the arterial blood-pressure, as the curve of the pressure 

 taken simultaneously remained practically 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 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 fibres) and pass to it in company with the splenic 

 artery. The nerve-cells from which they arise are in physiologic relation 

 with nerve-fibres (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 



