THE CENTRAL NERVOUS SYSTEM 767 



area of distribution without sending any communicating branches to 

 make junction with its neighbours. The consequence of these two 

 anatomical facts is : (i) that interference with the blood-supply of 

 the brain between the heart and the circle of Willis does not readily 

 produce symptoms of cerebral anaemia ; (2) that the blocking 

 of any of the arteries which arise from the circle or any of their 

 branches leads to destruction of the area supplied by it. The basal 

 ganglia are fed by twigs from the circle of Willis and the beginning 

 of the posterior, middle, and anterior cerebral arteries. Of these 

 the most important are the lenticulo-striate and lenticulo-optic 

 branches of the middle cerebral, which are given off near its origin, 

 and run through the lenticular nucleus into the internal capsule, and 

 thence to the caudate nucleus and optic thalamus respectively. The 

 chief part of the blood from the circle of Willis is carried by the 

 three great cerebral arteries over the cortex of the brain. The white 

 matter, with the exception of that in the immediate neighbourhood 

 of the basal ganglia, is nourished by straight arteries which penetrate 

 the cortex. The middle cerebral supplies the whole of the parietal 

 as well as that portion of the frontal lobe which lies immediately in 

 front of the fissure of Rolan'do and the upper part of the temporal 

 lobe. The rest of the frontal lobe is supplied by the anterior cere- 

 bral, and the occipital lobe, with the lower part of the temporal lobe, 

 by the posterior cerebral. The medulla oblongata, cerebellum, and 

 pons are fed from the vertebrals and the basilar artery before the 

 circle of Willis has been formed. 



Cerebro-spinal Fluid. The cerebro-spinal fluid, which fills the 

 ventricles of the brain and the central canal of the cord, is continuous 

 with that contained in the subarachnoid space through the foramen 

 of Magendie, an opening in the piece of pia mater that helps to roof 

 in the fourth ventricle. It is secreted in part by the cubical cells 

 covering the choroid plexus, a fold of pia mater which projects 

 into each lateral ventricle. It is a thin watery fluid, faintly alkaline 

 in reaction to litmus, and with a specific gravity of about 1010. It 

 contains the ordinary salts, a very small amount of proteids (usually 

 about *i per cent.), and a little glucose (Nawratzki). Its composition 

 is evidently very different from that of ordinary lymph. 



PRACTICAL EXERCISES ON CHAPTER 



i. Section and Stimulation of the Spinal Nerve-roots in the Frog. 

 Select a large frog (a bull-frog, if possible). Pith the brain. Fasten 

 the frog, belly down, on a plate of cork. Make an incision in the 

 middle line over the spinous processes of the lowest three or four 

 vertebrae, separate the muscles from the vertebral arches, and with 

 strong scissors open the spinal canal, taking care not to injure the 

 cord by passing the blade of the scissors too deeply. Extend the 

 opening upwards till two or three posterior roots come into view. 

 Pass fine silk ligatures under two of them, tie, and divide one root 

 central to the ligature, ths other peripheral to it. Stimulate the central 



