VASOMOTOR SUPPLY OF THE ORGANS. 



623 



important thing is the increase in the quantity of blood flowing 

 through. In the brain, owing to the pecuUarities of its position, 

 it has been suggested that perhaps no actual increase in size is 

 possible. It is evident, however, that the existence of the liquid 

 in the subarachnoidal space makes possible some actual expan- 

 sion of the organ. For as the pressure upon this liquid increases it 

 may be driven into the dural sac of the cord (Fig. 258) and along the 

 sheaths of the cranial and spinal nerves. To what extent this is 

 actually possible in man we do not know, nor do we know how much 

 cerebrospinal liquid is contained in the skull and brain of man. In 

 the dog Hill * finds experimentally that the brain can expand only 

 by an amount equal to 2 or 3 c.c. without causing a rise of intra- 

 cranial tension ; so that probably these figures represent the amount 

 of expansion possible in this animal by simple squeezing out of the 

 cerebrospinal liquid. If the rise of arterial pressure is such as to 

 expand the brain beyond this point, then it may not only force 

 out cerebrospinal liquid, if any remains, but, as explained in the 

 last paragraph, it will compress the veins and raise intracranial 

 pressure. To the extent that the veins are compressed as the ar- 

 teries expand no actual increase in the size or blood-capacity of the 

 brain takes place. That an expansion of the brain arteries com- 

 presses the veins is indicated very clearly by the normal occurrence 

 of a venous pulse in this organ. The blood flows out of the veins of 

 the brain in pulses synchronous with the arterial pulses, and this 

 venous pulse may be recorded easily as shown in Fig. 262. In this 

 case the sudden expansion of the arteries compresses the cerebral 

 veins, giving a synchronous rise of pressure in the interior of the 

 sinuses. Some authors (Geigel, Grashey), on purely theoretical 



Fi?. 262. — Simultaneous record of pulse in the circle of Willis (c) and in the torou- 

 lar Herophili (<)• The tracing from the circle of Willis was obtained by means of a 

 Hurthle manometer connected with the head end of the internal carotid. It will be noted 

 that the pulses are simultaneous, indicating that the venous pulse is due to the transmis- 

 sion of the arterial pulse through the bram substance. 



grounds, have held that this compression of the veins in cases of an 

 extensive rise in arterial pressure may result in a diminished 

 blood-flow through the organ,— a sort of self-strangulation of 

 its own circulation. Actual experiment shows that this is not the 

 case. Any ordinary rise of general arterial pressure is accompanied 



* Hill, "The Physiology and Pathology of the Cerebral Circulation," 

 London, 1896. 



