462 PHYSIOLOGY 



cerebral hemispheres and the third ventricle, sending prolongations into the 

 lateral ventricles. This layer of connective tissue is covered on one surface 

 by the ependyma of the ventricles, on the other surface by the ependyma 

 forming the roof of the third ventricle. It carries a rich plexus of blood 

 vessels known as the choroid plexus, and the ependyma covering the vascular 

 fringes which dip into the cerebral ventricles consist of clear columnar or 

 cubical cells, often spoken of as the epithelium of the choroid plexus. 

 Similar vascular fringes are found in the roof of the fourth ventricle. 



The pia mater is a layer of connective tissue which serves to carry the 

 blood-supply to the whole surface of the brain. It is closely applied to the 

 surface and follows all the irregularities of the latter, dipping down into the 

 various fissures and crevices on the brain. In the spinal canal the pia mater 

 sends out a series of processes on each side of the spinal cord, the ligamentum 

 denticulatum, the outer extremities of which are attached to the dura 

 mater and serve to sling the spinal cord in its dural sheath. 



The brain is richly supplied with blood. Its chief supply is derived from 

 the two carotids and the two vertebral arteries. The vertebrals unite on 

 the lower surface of the bulb to form the basilar artery, which divides again 

 at the anterior extremity of the pons varolii into two branches which unite 

 with the two carotid arteries to form the circle of Willis, so that the pressure 

 in this arterial circle can be maintained indifferently by any three out of the 

 four arteries by which it is supplied. From these vessels three main arteries, 

 the anterior, middle and posterior cerebral, pass up to supply the correspond- 

 ing regions of the outer surface of the brain, while the inner parts of the 

 brain, e.g. the corpus striatum, optic thalamus, &c., are supplied from 

 arteries arising from the circle of Willis and passing straight into the sub- 

 stance of the brain. The connection between the vascular supply of the 

 different parts of the brain is slight and effected only by the capillaries ; 

 hence obstruction of any one vessel, such as the middle cerebral, perma- 

 nently cuts off the blood- supply to the greater part of the area supplied by 

 it and the result is death and softening of the brain substance. The arteries 

 supplying the surface of the brain divide up into arterioles and capillaries 

 within the pia mater, and the capillaries run into the brain substance sur- 

 rounded by a so-called lymphatic sheath, which apparently communicates 

 with the subarachnoid space. 



In certain cases of disease these perivascular sheaths may be found to 

 contain leucocytes often filled with products of disintegration of the nervous 

 tissues. 



THE CEREBRO-SPINAL FLUID 



The subarachnoid space contains a thin transparent colourless fluid, 

 known as the cerebro-spinal fluid. In composition this fluid resembles 

 blood plasma minus its protein constituents. It contains a mere trace of 

 coagulable proteins but it has the same molecular concentration as the blood 

 plasma and its salts are identical with those of the blood plasma. It also 

 contains other diffusible constituents of blood plasma, e.g. small traces of 

 sugar and of urea. It may be collected by introducing a cannula through the 



