144 



DISTRIBUTION OF THE BLOOD. 



100- DISTRIBUTION OF THE BLOOD. In the rabbit, one fourth of the 

 total amount of the blood is found in each of the following :- a, in the passive 

 muscles : &, in the liver ; c, in the organs of the circulation (heart and great 

 vessels) ; d, in all other parts together. 



Methods. The methods adopted do not give exact results. J. Ranke ligatured the parts 

 during life,* removed them, and investigated the amount of blood while the tissues were still 

 warm. 



Influencing Conditions. The amount of blood is influenced by (1) the anatomical dis- 

 tribution of the vessels (vascularity or the reverse) as a whole ; (2) the diameter of the vessels,. 

 which depends upon physiological causes (a) on the blood-pressure within the vessels ; (b) on 

 the condition of the vaso-motor or vaso-dilator nerves ; (c) on the condition of the tissues them- 

 selves, e.g., the vessels of the intestine during absorption ; by the vessels of muscle during 

 muscular contraction ; of vessels in inflamed parts. 



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 </r organ goes hand in hand 

 with the variation* in its states of activity. 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 nearly 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. Ranke). Iu the condition of increased activity, a more 

 rapid renewal of the blood seems to occur ; after muscular exertion the duration of 

 the circulation diminishes (Vierordt). 



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 (Glcy). 



Age. The development of the heart and large vessels determines a different distribution of 

 the blood in 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, 



Fig. 118. 

 Mosso's plethvsmograph. G, glass-vessel for holding a limb ; F, flask for varying the water- 

 pressure in G ; T, recording apparatus, 

 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. In order to estimate and register the amount 

 of blood in a limb Mosso devised an instrument (fig. 118), which he termed a 

 plethysmograph. 



