THE CEREBRUM 611 



teristics of physiologic interest. These vessels consist of large sinuses formed 

 by folds of the dura mater or, as at the base of the cranium, by the dura mater 

 and the bone. These sinuses, from the very nature of the tissues which 

 enter into their formation, have rigid walls and will therefore withstand any 

 pressure to which they may be subjected under physiologic conditions. The 

 same obtains at their points of exit from the cranium where a free outflow 

 is in consequence always assured. 



The various sinuses have opening into them, the veins which return the 

 blood from the cortex and subjacent white matter, and from the inner struc- 

 tures of the brain. Neither sinuses nor veins have valves and most of the 

 veins which empty into the superior longitudinal sinus have their mouths 

 directed forward, hence the blood discharged from these veins must flow 

 against the current in the sinus. The venous blood leaves the cranium 

 mainly by way of the internal jugular veins which are direct continuations of 

 the lateral sinuses. 



The Intra-cranial Lymph-spaces. In order to understand the phe- 

 nomena attending the circulation of blood through the cranium it is necessary 

 to take into consideration an important fact, viz. : that the brain and spinal 

 cord are surrounded on all sides by a relatively large and continuous lymph- 

 space. This space which is found between the arachnoid and the pia mater 

 is filled with a liquid, the so-called cerebrospinal fluid, which being interposed 

 between the brain and the skull on the one hand and the spinal cord and the 

 vertebrae on the other hand, acts as a water cushion protecting these delicate 

 organs from the injury which might result from sudden jars. The ventricles 

 of the brain are also filled with cerebrospinal fluid which is in communication 

 with that in the subarachnoid space through the foramen of Magendie and the 

 foramina of Key and Retzius. The cerebrospinal fluid may also penetrate 

 into the perineural lymph-spaces surrounding the cranial and spinal nerves. 

 The quantity of the cerebrospinal fluid is relatively small, amounting to 

 from 60 to 80 c.c. 



The Mechanism of the Intra-cranial Circulation. As previously 

 stated, by virtue of the physical relations existing between the blood, the 

 brain, the cerebrospinal fluid and rigid walls of the cranium, the flow of the 

 blood through the brain and cranial cavity, is attended by certain phenomena 

 which are peculiar to this region and present in no other situation. 



Taking as a point of departure the condition of the arteries during the 

 cardiac diastole, the relations of these structures are somewhat as follows: the 

 cerebrospinal fluid occupies all the available lymph-space, but under a pres- 

 sure approximately equal to that in the large veins and hence not materially 

 above that of the atmosphere; the pressure in the arteries, capillaries and 

 veins presents the usual values in these different regions of the vascular 

 apparatus; the brain presents a volume which may be termed diastolic. 



With the occurrence of the succeeding cardiac systole, the cerebral 

 vessels, receiving an additional volume of blood, expand and occasion a 

 corresponding increase in the volume of the brain, which is accomplished by 

 a partial displacement of the cerebrospinal fluid into extra-cranial lymph- 

 spaces. Because of the fact that the displacement of the cerebrospinal 

 fluid is insufficient to permit of the complete expansion of the brain, there is 

 developed in the intra-cranial lymph-spaces a counter-pressure (the so-called 

 intra-cranial pressure) which would keep pace with and finally equalize the 



