676 Dynamic Theory. 



mals, is confirmed by the effects of lesions and diseases of these organs. 

 Many cases have been observed in which diseases of vision were found 

 to be due to lesions of the occipital lobe. A common cause of a partial 

 loss of memory is embolism, or thrombosis. That is, an obstruction, 

 usually in the form of a coagulated clot, stops up a small artery in the 

 brain, thus cutting off the blood supply of all the cells which lie be- 

 yond the obstruction, and which depend on that supply for materials to 

 repair the waste caused by the work done through them. The arteries 

 divide and subdivide as they pass outward from the great aorta, like the 

 branches of a tree, so that a clot which readily passes along the larger 

 branches will finally reach a twig too small for it to squeeze through. 

 The brain cells beyond the obstruction will at once be- 

 come functionless, and all the memories depending on 

 their re-erection will be lost until the obstruction be re- 

 moved. The larger the arterial twig is which is stopped 

 up, the larger is the tract of injured memories; thus a 

 plug at ( a ) will result in greater mischief than if it 

 FIG. 368. should get on to (b). Abcess, ecpiesma or fracture 

 with compression of the brain, effusion, &c. , are attended by injury to 

 some of the sensory or motor memories. A sudden blow upon the 

 head ma}' produce momentary compression of the braia, with sudden 

 and temporary loss of all memory. 



As mentioned in last chapter, the motor speech memories are located 

 in the Island of Reil. This is smooth in most animals, but in man and 

 the apes it is convoluted, and also to some extent in the porpoise. (See 

 fig. 267. ) The principal organs concerned in speech, are single and 

 median, which may perhaps account for the fact that they are usually 

 operated by the speech organ of one side only. In right-handed per- 

 sons that is the left side, and in left-handed persons usually the right 

 side. The faculty of ordinary speech involves, first, the memory of 

 words ; that is, the conception of sounds as pictured in the memory, 

 and their perception as associated with definite ideas ; and, second, the 

 memory of the necessary movement of the muscles in the articulation. 

 The first part belongs to the department of hearing. The second, or 

 motor part of the faculty has its seat in the insula, or Isle of Reil. 

 Both parts receive their blood supply and nourishment from the middle 

 cerebral artery, so that injury to this is liable to effect the memory of 

 language, or its articulation, or both. In reading, the sight memories 

 are involved. The reader must have memories in his sight tract in the 

 posterior lobe, of the shapes and appearance of printed words in asso- 

 ciation with the faculty of the muscular movements necessary to their 

 articulation. The hearing memories may be atrophied, and yet the per- 

 son be able to read from a book, as deaf persons can often do. In writ- 



