THE BRAIN. 



THE BRAIN. 



The affections of the brain of most anatomical interest are those involving its 

 circulation, the motor areas, and the motor paths. Paralyses may arise from (#) 

 interference with the motor areas in the cortex by hemorrhages, injuries, or tumors; 

 (b) destruction of the motor paths from the cortex to their point of exit from the 

 brain; (V) injury of the nerves at their exit from the brain. 



Disturbances of the circulation may be either of the nature of anaemia or ischae- 

 mia, causing softening, or of congestion, causing apoplexy. 



Tumors of the brain interfere with the functions of the part in which they are 

 located, as do also wounds. In all of these a knowledge of brain localization is 

 essential. 



THE CIRCULATION OF THE BRAIN. 



The blood reaches the brain by means of the two internal carotid and the two 

 vertebral arteries. The vertebrals enter through the foramen magnum and unite to 



A. communicans anterior 



A. cerebri anterior 



A. carotis interna 



A cerebri media 



A. communicans posterior 



A. anterolateral 



A. choroidea 



A. cerebri posterior 



A. basilaris 



A. vertebralis 



FIG. 29. Circulus arteriosus or circle of Willis. 



form the basilar, which at the upper border of the pons divides into the two pos- 

 terior cerebrals. These give off two small branches which go to the internal carotids ; 

 they are the posterior communicating arteries. The carotids divide into the anterior 

 and middle cerebral arteries, the anterior communicating with one another by means 

 of the anterior communicating artery. Thus we have the circle of Willis {circuhis 

 arteriosus), formed by the posterior cerebral, posterior communicating, internal car- 

 otid, anterior cerebral, and anterior communicating arteries on each side. 



The blood supply of the brain is divided into an anterior division, furnished by 

 the carotids, and a posterior division, supplied through the basilar and posterior 

 cerebrals. The communication branch running between these two sets of vessels is 

 so small that if either is occluded the supply of blood is practically cut off from that 

 point and ischaemia results, at least in most cases. 



The anterior division is subdivided into a right and a left half by the two carotid 



