DISTRIBUTION OF BLOOD IN THE BODY 241 



The blood flow to the brain calls for a special discussion. In the child 

 while the skull is not yet completely ossified, the great fontanel exhibits pulsa- 

 tions which are undoubtedly caused by the heart beats and by the respiratory 

 movements, and which show moreover that the blood supply to the brain may 

 vary under different circumstances. 



How far this is true in the mature, uninjured skull has been much debated. 

 Were the skull cavity rigidly closed on all sides, and were the brain substance 

 nearly incompressible, the same quantity of blood should be present in the brain 

 at all times. But this would not be true, if water or other material were secreted 

 from the blood vessels or otherwise extravasated ; for then the quantity of blood 

 equivalent to the volume of material poured out of the vessels would be dis- 

 placed. Otherwise blood flowing away by the veins would always make room for 

 the blood flowing in by the arteries. 



But it has been shown that this conclusion is not strictly correct, and that 

 the quantity of blood in the brain can in fact increase and decrease. The skull 

 cavity is not surrounded on all sides by solid bony walls. It communicates with 

 the spinal canal, between the inner surface of which and the outer surface of 

 the dural sac are numerous venous plexuses connected with the veins of the 

 general system. The foramina intervertebralia are filled with a vacuolated tissue, 

 which can be pressed outward. The subdural space communicates with the deep 

 lymph vessels and glands of the neck, as well as with the lymph tracts of the 

 peripheral nerves. 



The subarachnoid spaces are likewise in connection with the lymph tracts 

 of the peripheral nerves. The cerebrospinal canal therefore must be regarded as 

 a rigid-walled cavity with an elastic door. 



Now it has been found both by experiments on animals, and by physical 

 (Grashey) and mathematical (Lewy) calculations, that the regulation of the 

 blood flow to the brain takes place in exactly the same way as in the other 

 organs i. e., dilatation of the arteries produces an increase in the flow, con- 

 striction a diminution. Any stasis of venous blood causes an arterial anemia, 

 just as does a severe compression, as for example by a foreign body forced into 

 the skull cavity. So long, therefore, as it is a question only of the alterations 

 in arterial volume, which correspond to the physiological needs, the circum- 

 stance that the brain is inclosed by a solid, unyielding capsule is of no essential 

 importance. 



Jensen has found that the quantity of blood flowing to the brain of a rabbit 

 is on the average 136 c.c. per 100 g. per minute (extremes 60-278 c.c.). In the 

 dog he found as a mean of two researches 138 cc. The brain receives relatively 

 more blood than any of the other organs thus far studied except the thyroid gland. 



REFERENCES. R. Tiegerstedt, " Lehrbuch der Physiologic des Kreislaufes," 

 Leipzic, 1893. 



