CHAP, v:] NUTRITION. 433 



arrangements of the two organs. In the kidney the small arteries 

 are relatively numerous, and a large portion of the blood in the 

 kidney is contained in them ; in the spleen the small arteries are 

 relatively few, and the great bulk of the blood is contained in the 

 capillaries and in the meshes of the peculiar splenic tissue. Con- 

 sequently the blood-flow through the spleen is of a more even 

 character than that through the kidney, and the effects of variations 

 in the distension of the arteries, and of vaso-motor influences gene- 

 rally are less directly felt. 



MWIilW^^ 



Fia. 66. NORMAL SPLEEJI CURVE FROM DOG. 



The upper curve is the spleen curve shewing the rhythmic contractions and 

 expansions ; the smaller waves are due to the respiratory movements. The lower 

 curve is the blood-pressure curve and the point a of the spleen curve corresponds in 

 time to the point 6 of the blood-pressure curve. The marks on the tune curve 

 below indicate seconds. 



Besides the respiratory undulations the spleen curve usually 

 shews, as seen in the figure, large slow variations of volume. 

 Rhythmic contractions and expansions, though not always present, 

 frequently make their appearance, each contraction with its fellow 

 expansion lasting in the cat and dog about a minute, and recurring 

 with great regularity for a long time. There can be little doubt but 

 that these variations in volume are due to rhythmic contractions, 

 with intervening relaxations, of the muscular trabeculse and capsule. 

 In many animals the contractility of the splenic tissue is shewn by 

 the white lines of constriction which appear when the electrodes 

 of an induction machine in action are drawn over its surface ; and 

 similar lines may be produced by mechanical stimulation with the 

 point of a needle. So that the spleen may be considered as a 

 muscular organ, now expanding to receive a larger quantity of 

 blood and now contracting to drive the blood on to the liver. We 

 have evidence moreover that the muscular activity of the spleen is 

 under the dominion of the nervous system. A rapid contraction of 

 the spleen may be brought about in a direct manner by stimulation 

 of the splanchnic or vagus nerves, or in a reflex manner by stimu- 

 lation of the central ends of a sensory nerve ; it may also be 

 p. 28 



