788 



THE NERVOUS SYSTEM. 



of the posterior roots. Branches of these arteries anastomose in the pia, and from this 

 pial network branches pass directly into the substance of the cord. The largest, a 

 branch of the anterior spinal, passes into the pia of the anterior fissure, and penetrating 

 the cord, is distributed to the gray matter of the anterior horn, and to the median gray 

 matter as far back as Clarke's column. Smaller arteries from the posterior spinal supply 

 the posterior regions of the cord. These arteries, like those in the brain, are terminal 

 in the sense that, while anastomosing freely in the pia, after penetrating the substance 

 of the cord there are no further anastomoses. 



HYPER^MIA AND ANEMIA. 



The appearance of the brain tissue after death does not always fur- 

 nish reliable indications of its blood contents during life, though it is 

 perhaps more to be depended on than the ap- 

 pearance of the meuiuges. 



Some of the more common conditions deter- 

 mining hypercemia which are mentioned above as 

 influencing the meuinges apply also to the sub- 

 stance of the brain. 



Active hyperaemia may occur in various in- 

 flammatory conditions of the brain. Hyperseinia 

 of the brain is quite common in deaths from in- 

 solation and after conditions accompanied by 

 acute delirium. Passive hyperaemia may occur 

 in conditions similar to those which determine 

 congestions in other organs of the body, such as 

 chronic diseases of the heart or lungs. It may 

 be induced by anything which prevents venous 

 return, such as iutracranial tumors which com- 

 press the sinuses, or by thrombosis. 



In sections of hyperaemic brains the small 

 blood points from the cut ends of small vessels 

 are more numerous and conspicuous than under 

 normal conditions, and the brain tissue, partic- 

 ularly the gray matter, may have a diffuse red 

 color. If excessive, the convolutions may be 

 somewhat flattened, the brain tissue and pia 

 mater may be redeniatous, and the ventricles contain fluid, 

 tion of the vessels may be general or localized. 



The most constant lesion of what is known as caisson disease, a con- 

 dition resulting from exposure to sudden changes in atmospheric press- 

 ure, is a congestion of the brain, cord, and meninges. Areas of soften- 

 ing in the brain and cord are not infrequent and there may be effusion 

 into the meninges. 



Anaemia of the brain may be either local or general. It may depend 

 upon a general anaemia or upon general disturbances of the circulation, 

 such as mitral stenosis or regurgitation ; or upon local interference with 

 the arterial blood supply, such as complete or partial obstruction of the 



FIG. 517. DIAGRAM OF THE 

 CIRCLE OF WILLIS AND ASSO- 

 CIATED VESSELS. 



a. Anterior cerebral; b, an- 

 terior communicating ; c, inter- 

 nal carotids; rf, middle cere- 

 bral ; e, posterior communicat- 

 ing: /, posterior cerebral; y. 

 basilar ; ft, vertebral. 



The couges- 



