PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 253 



Intracranial Pressure 



One word more with regard to what is known as intracranial pressure, 

 that is, the pressure in the space between the skull and the brain. 

 Under ordinary conditions it must be equal to that in the cerebral capil- 

 laries, and may be measured by connecting a sensitive manometer with 

 a tube screwed into the cranium as described above. It has been found 

 to vary from mm. Hg in a man standing erect to 50-60 mm. Hg in a 

 dog poisoned by strychnine. It becomes increased, not only by com- 

 pression of the veins of the neck and by an increase in general arterial 

 pressure, but also in pathological conditions, such as hydrocephalus. A 

 new growth in the brain, if it occupies more space than the tissue which 

 is destroyed, exerts pressure on all parts of that region of the cranial 

 cavity, but this pressure may not be transmitted equally throughout 

 the cranial contents, for the falciform ligaments and the tentorium sup- 

 port a part of it, thus directing the spread of pressure along certain 

 pathways. The structures at the base of the brain, the optic nerves, 

 the veins of Galen and the Sylvian aqueduct are most affected in this 

 way. If the pressure is rapidly applied, however, it may rise through- 

 out the cranial contents. In such cases the pressure is, of course, cir- 

 culatory in origin, since immediately after death from cerebral tumor 

 the intracranial pressure is not found to be raised. 



The major symptoms of cerebral compression are no doubt due to 

 anemia of the medulla oblongata, which may be the result either of 

 pressure applied locally in the bulbar region, where the presence of a 

 very small foreign body or only trivial tumor formation is sufficient to 

 destroy life, or of pressure transmitted from the cerebral cavity, in 

 which case, on account of the support offered by the tentorium, a much 

 larger growth is required to affect the medulla. Internal hydrocephalus 

 produced by blocking of the aqueduct of Sylvius and the veins of Galen 

 causes the greatest rise in intracranial tension, and may affect the me- 

 dulla, because the brain is driven downwards so as to pinch the bulb 

 against the occipital bone. It must be emphasized that it is not the 

 pressure per se that causes the symptoms, but the attendant anemia, 

 the symptoms of acute cerebral anemia and of compression being iden- 

 tical (Leonard Hill 19 ). To relieve the compression, trephining is the 

 common practice. The trephine hole should be as large and as near 

 to the source of compression (tumor, etc.) as possible. 



CIRCULATION THROUGH THE LUNGS 



The pulmonary or lesser circulation, as it is called, is quite different 

 from the systemic circulation. In the first place, because the pressure 



