560 TEXT-BOOK OF PHYSIOLOGY. 



The Intra-cranial Circulation. The circulation within the cranium 

 presents certain peculiarities which distinguish it from that in other parts of 

 the body. These peculiarities reside in part in the anatomic arrangement of 

 the blood-vessels, in the probable absence of vaso-motor nerves to the blood- 

 vessels, and in greater part in the fact that the brain and its blood-vessels 

 are contained in a case with rigid, unyielding, and closed walls. 



The Blood-supply. As stated in a previous paragraph the arteries 

 supplying the brain with blood are four in number, viz. : the two internal 

 carotids and the two vertebrals. These four arteries anastomose very freely 

 at the base of the brain, the anastomosis constituting the circle of Willis. 

 From this circle there arise the anterior, middle and posterior cerebral arteries 

 which are distributed to the cortex and the underlying white matter. The 

 basal ganglia, the capsule and adjacent white matter are supplied by a num- 

 ber of branches which arise from the circle of Willis or from the three cerebral 

 arteries immediately after their origin. From the distribution of these two 

 sets of vessels they have been named the cortical and the central ganglionic 

 respectively. 



The venous blood is returned by a system of vessels which present charac- 

 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. 



