THE BHAIN OF MAMMALS AND THE OLFACTORY APPAEATUS. 



225 



the brain, embracing the two central gyri, after disease or injury of which disturb- 

 ances in motility almost always appear on the opposite side of the body. These 

 disturbances are divided into phenomena of irritation and phenomena of degenera- 

 tion. The symptoms of irritation are expressed by spasms or convulsive movements; 

 the symptoms of degeneration by a more or less high degree of inability to set the 

 muscles in motion at will, oftentimes only in a sense of weakness, or by awkward- 

 ness in executing complicated movements. 



From an accurate analysis of the known cases of disease, it may be stated that, 

 in disease of the upper part of both central gyri and of the paracentral lobule, the 

 motor disturbances manifest themselves preponderantly in the legs; that, if the 

 inferior end of the central gyri is involved, the regions of the facialis and the hyp- 

 oglossus are affected, and that motor disturbances in the upper extremity especially 

 may be produced by disease of about the middle, and a portion of the upper, third 

 of these gyri. The separation of the "centers" from one another is not a distinct one. 



Complete destruction of separate parts of the central gyri may lead, in man, to 

 permanent paralysis of the muscles connected with those parts. The paralyzed mus- 

 cles almost always fall into a state of contracture. 



Fig. 149. — The cortical areas, as far as they are demonstrable by 

 irritation. A, Cat. B, Rabbit. (After Mann.) 



Diseases or injuries that involve the cortex of the left inferior frontal gyrus or 

 the left insula generally lead to a more or less complete loss of speech, although the 

 vocal organs may still be normally innervated and the patient may often perfectly 

 understand all that is said. It appears that the ability to understand whatever is 

 said even in a loud voice is lost if the superior temporal gyrus is affected. The 

 ability to comprehend reading has been repeatedly seen to be lost after lesions of the 

 cortex situated between the apex of the occipital lobe and the posterior end of the 

 Sylvian fissure. Perhaps deep tracts are here involved, and it is not a question of 

 cortical localization. 



Lesions in the region of the occipital lobe may lead to disturbance of vision, 

 manifested as a dimness of vision or blindness on the outer side of the eye on the 

 aflfected side and on the inner side of the other eye (see below). 



The preservation of the cuneus appears to be especially important for the com- 

 prehension of what is seen. 



Sensibility may also suffer from affections of the cerebral cortex. Feelings of 

 numbness, heaviness, and marked disturbances of the muscular sense are frequently 



