THE NERVOUS SYSTEM. 787 



The atrophy affects principally the cerebral hemispheres, and may be 

 uniform or more marked in some parts than in others, involving the 

 whole of a hemisphere or of a lobe or only single convolutions or groups 

 of these (Fig. 516). The convolutions are small, the sulci broad, the 

 ventricles usually dilated, the brain tissue is firm, the gray matter discol- 

 ored, the white substance grayish in color; the blood-vessels may be 

 dilated. The basal ganglia may be small. Serum accumulates- in the 

 pia mater and in the ventricles; the pia mater, and often the skull, be- 

 come thickened; the brain tissue may be cedeinatous or contain small 

 haemorrhages. The nerve elements of the brain tissue are those most 

 involved in the atrophy, the diminished areas being usually harder and 

 firmer than normal. 



PIGMENTATION OF THE BRAIN. 



This may occur in any portion of the brain or its meninges from the 

 decomposition of extravasated blood. In persons affected by malaria 

 the gray matter of the brain has sometimes a dark or even blackish ap- 

 pearance. This color is due to the presence of black pigment granules 

 within the capillary blood-vessels. The obstruction to the vessels by 

 masses of these pigment granules may cause capillary apoplexies. The 

 pigment may also be found in the walls and in the lumina of the vessels 

 of the pia mater. 



Pigment patches of congenital origin are not infrequently seen in the 

 pia mater. They may be due to the presence of branching pigniented 

 cells. 



CIRCULATORY AND VASCULAR CHANGES IN THE BRAIN AND 

 SPINAL CORD. 



Anatomical Considerations. 



In studying the circulatory changes in the brain and cord certain peculiarities in 

 that circulation should be borne in mind. The vessels which nourish the brain arise 

 from a remarkable anastomosis of large arterial trunks at its base, known as the circle 

 of Willis (see diagram Fig. 517). From this "circle" pass off to each hemisphere three 

 main branches the anterior, the middle, and the posterior cerebral. These arteries 

 ramify in the pia, where they anastomose freely. From this anastomosis small branches 

 are given off which penetrate the brain substance, the shorter breaking up into capil- 

 laries in the gray matter, the longer passing to the underlying white matter. After 

 entering the cortex there is no further anastomosis, the capillaries of a cortical artery 

 passing directly over into a venous system of capillaries without communicating with 

 capillaries of other arteries. They are thus " terminal arteries." In addition to these 

 arteries, which supply the hemispheres, branches from the circle of Willis are distrib- 

 uted to the basal ganglia. These arteries are much larger than those which pass from 

 the pia into the cortex, and beyond the circle of Willis do not form anastomoses with 

 one another. Thus it is that occlusions of the arteries supplying the basal ganglia are 

 much more serious, aside from the importance of the parts involved, Xhan of those pass- 

 ing to the cortex. 



Three main arteries furnish blood to the cord: the anterior spinal, lying along the 

 opening of the anterior median fissure, and two posterior spinal, lying near the entrances 



