THE CIRCULATION THROUGH SPECIAL ORGANS 441 



the periosteal and meningeal layers of the dura mater. In many places 

 their channel is hollowed out in the bone, their patency being assured 

 in addition by numerous delicate threads of dura mater fastened to 

 their external surface. In accordance with their location, these 

 sinuses collect the blood not only from the cortical and ganglionic 

 veins of the cerebrum, but also from the enveloping membranes and 

 the bones. Moreover, those situated at the base receive at least a 

 part of the blood of the orbital cavities and the eyeballs. 



Intracranial Pressure. As has been stated above, the cranial 

 cavity forms a natural plethysmograph for the brain. If a cannula 

 is inserted in a' trephine opening and is connected with a recording 

 tambour, two types of oscillations will be registered, the smaller ones 

 being caused by the action of the heart and the larger ones by the res- 

 piratory movements. They may be rendered more conspicuous by 

 incising the dura, because this membrane places a certain resistance 

 in the path of the expanding brain. In infants, these pulsations 

 may be observed in the region of the fontanelles, and in adult persons 

 through accidental defects in the skull. 1 The question, whether the 

 cerebral blood-vessels are also expanded when the skull plates are 

 intact, has been answered positively by Bonders 2 and Schultze 3 who 

 have observed the brain through a piece of glass firmly fixed and sealed 

 in a trephine opening. It seems that an interchange of pressure is 

 still possible in spite of the fact that the brain is situated, so to speak, 

 in a compartment possessing perfectly rigid walls. Under normal 

 conditions, the place of least resistance is the occipitoatlantal mem- 

 brane, but a slight interchange of pressure may also be effected through 

 the carotid foramina and the points of exit of the cranial nerves. 



At all events, it must be evident that the brain cannot undergo 

 more than a very limited alteration in its volume. In the dog, for 

 example, an expansion of only 2 to 3 c.c. is possible. A greater in- 

 crease is invariably associated with a rise in the intracranial pressure 

 and a compression of the cerebral veins. The interchange of pressure, 

 made necessary by such slight volumetric variations as are produced by 

 the systolic movements of the heart, is easily effected by an encroach- 

 ment upon the venous blood current. Thus, we actually find that the 

 distal venous channels pulsate synchronously with the arteries. 

 Greater expansions of the brain are made possible by a displacement 

 of the cerebrospinal fluid. A certain yielding is also had at the fora- 

 mina intervertebralia, where the loose tissue is pressed outward when- 

 ever the cerebral fluid is subjected to an undue pressure. The tissue 

 may also be made to give way slightly at the other cranial orifices. 

 In the second place, we may obtain an actual transfer of the cerebral 

 fluid into the lymph spaces of the cord or into the lymphatic channels 

 of the neck, orbital cavity, internal ear, and cranial nerves. Further- 



1 For historical data see Hill, The Cerebral Circulation, London, 1896. 



2 Onderzoekingen ged. in het phys. Lab. d. Utrechtsche Hoogeschool, 1850. 



3 Med. Zeitschr., St. Petersburg, 1866. 



