PLETHYSMOGRAPHY. 197 



Activity of an Organ. The most important factor, however, is the 

 state of activity of the organ itself ; hence, the saying, " ubi irritatio, ibi 

 affluxus." We may instance the congestion of the salivary glands 

 and the gastric mucous membrane during digestion, and the increased 

 vascularity of muscles during contraction. As the activity of organs 

 varies at different times, the amount of blood in the part or organ goes 

 hand-in-hand with the variations in its states of activity (J. Ranke). 

 When some organs are congested others are at rest; during digestion, 

 there is muscular relaxation and less mental activity : violent muscular 

 exertion retards digestion during great congestion of the cutaneous 

 vessels the activity of the kidneys diminishes. Many organs (heart, 

 muscles of respiration, certain nerve-centres) seem always to be in a 

 uniform state of activity and vascularity. 



During the activity of an organ, the amount of blood in it may be 

 increased 30 per cent., nay even 47 per cent. The motor organs of 

 young muscular persons are relatively more vascular than those of old 

 and feeble persons (J. Eanke). 



During a condition of mental activity, the carotid is dilated, the dicrotic wave 

 in the carotid curve is increased (the radial shows the opposite condition), and the 

 pulse is increased in frequency (Gley). 



In the condition of increased activity, a more rapid renewal of the 

 blood seems to occur; after muscular exertion the duration of the cir- 

 culation diminishes (Vierordt). 



3. The development of the heart and large vessels determines a different dis- 

 tribution of the blood iu the child from that which obtains in the adult. The heart is 

 relatively small from infancy up to puberty, the vessels are relatively large ; while 

 after puberty the heart is large, and the vessels are relatively smaller. Hence, it 

 follows that the blood-pressure in the arteries of the systemic circulation must be 

 lower in the child than in the adult. The pulmonary artery is relatively wide in 

 the child, while the aorta is relatively small ; after puberty both vessels have 

 nearly the same size. Hence, it follows that the blood-pressure in the pulmonary 

 vessels of the child is relatively higher than that in the adult (Beneke). 



101. Plethysmography. 



Plethysmograph. In order to estimate and register the amount of 

 blood in a limb Mosso devised an instrument (Fig. 87), which he 

 termed a Plethysmograph. It is constructed on the same principle as 

 the less perfect apparatus of Chelius and Fick. 



It consists of a long cylindrical glass-vessel, G, suited to accommodate a limb. 

 The opening through which the limb is introduced is closed with caoutchouc, and 

 the vessel is filled with water. There is an opening in the side of the vessel in 

 which a manometer tube, filled to a certain height with water, is fixed. As the 

 arm is enlarged with the increased supply of arterial blood passing into it at each 



