59 8 TEXT-BOOK OF PHYSIOLOGY 



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

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

 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 th< 

 veins which empty into the superior longitudinal sinus have their mouth? 

 directed forward, hence the blood discharged from these veins must flo^ 

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

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

 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 necessar) 

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

 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 mate] 

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

 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 communicatior 

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

 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 tc 

 from 60 to 80 c.c. 



The Mechanism of the Intra-cranial Circulation. As previousl) 

 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 materiall) 

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

 veins presents the Usual values in these different regions of the vasculai 

 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 

 rising pressure in the arteries. In consequence of this, the brain tissue, it is 

 believed, would be subjected to a pressure sufficiently great to interfere with 



