452 THE METABOLIC PROCESSES OF THE BODY. 



sometimes not visible. In other words, the spleen does not expand with 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, showing 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. 



Of special interest are the large slow variations of volume which, besides 

 the respiratory undulations, the spleen curve usually shows, as seen in the 

 figure. 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 regu- 



FIG. 117. 



IwfliyiiiiiuiiJiii uf'iiiiiii 



Normal Spleen Curve from Dog. (Roy.) The upper curve is the spleen curve showing 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. 



larity 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 contractions, with intervening 

 relaxations, of the muscular trabeculse and capsule ; the slower variations 

 are also probably due to the same cause. In many animals the contractility 

 of the splenic tissue is shown by the white lines of constriction which appear 

 when the electrodes of an induction machine in action are drawn over its 

 surface; and similiar lines may be produced by mechanical stimulation with 

 the point of a needle. So that the spleen in these animals 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. When the muscular 

 elements are scanty in or absent from the capsule and trabeculse, the expan- 

 sion and contraction of the whole organ must depend alone or chiefly on 

 variations in the width of the supplying arteries. We have evidence, more- 

 over, that the muscular activity of the spleen, whether of the muscular cap- 

 sule and trabeculse and arteries combined, or of the latter alone, 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 stimulation of the central end of a sensory 

 nerve ; it may also be caused by stimulation of the medulla oblongata with 



