CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 741 



respiratory movements ; and these, always very slight, are some- 

 tiiiics not visible. In other words, the spleen does not expand 

 wit h the increase of blood-pressure occurring in the splenic arteries 

 after each heart-beat ; this may be due to the muscular coat 

 resisting expansion. Moreover when the supply of blood to the 

 spleen is wholly and suddenly cut off, as by clamping the aorta, 

 the spleen curve sinks very slowly, shewing that the spleen is 

 diminishing in volume not suddenly but very slowly. The path- 

 way of the blood through the splenic reticulum is peculiar ; and 

 increase or decrease in the volume of the spleen means more or 

 less blood held in the spleen pulp, not necessarily a greater or 

 less flow of blood through the organ. 



fe: 



FIG. 95. NORMAL SPLEEN CURVE FROM DOG. (EoY.) 



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 b of the blood-pressure curve. The marks on the time curve 

 below indicate seconds. 



Of special interest are the large slow variations of volume 

 which, besides the respiratory undulations, the spleen curve usually 

 shews, as seen in the figure. Rhythmic contractions and ex-l 

 pansions, though not always present, frequently make their appear 4 

 ance, each contraction with its fellow expansion lasting in the cat 

 and dog about a minute, and recurring with great regularity for a 

 long time ; and besides these the volume varies widely from time 

 to time. There can be little doubt but that the rhythmic 

 variations in volume are due in these animals to rhythmic con- 

 tractions, with intervening relaxations, of the muscular trabeculre 

 and capsule ; the slower variations are also probably due to the 

 same cause. 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 in 



