THE CEREBRUM. 270 



give to the under surface of the pia mater a woolly 

 appearance. 



THE CEREBRUM. 



The cerebrum, arranged in two hemispheres, the 

 right and the left, constitutes the great mass of the 

 brain, weighing about seven and a half times as much 

 as the combined weights of the cerebellum, pons, and 

 the medulla. The two hemispheres are divided by a 

 deep fissure, the longitudinal, which lodges the falx 

 cerebri, connected by a great transverse commissure, 

 the corpus callosum. Each hemisphere presents for 

 examination an outer, convex, convoluted surface; an 

 inner, vertical, flat, convoluted surface; and an irregular 

 base. On the outer surface of the brain are a num- 

 ber of rounded elevations, disposed in a linear manner. 

 They are called the convolutions, and are separated by 

 sulci or depressions. 



A convolution is an elevation of the cortical layer 



* 



of the brain, caused by overgrowth. In the foetus the 

 convolutions are indicated by slightly-marked linear 

 elevations, separated by faint depressions, the future 

 sulci. The cortex develops and grows with increasing 

 rapidity, and puckers up over the subcortical stem of 

 white matter, forming the folds or convolutions of the 

 cortex. The convolutions are not strictly alike in both 

 hemispheres of the brain. Certain ones are more or 

 less perfectly and symmetrically developed. These are 

 designated primary convolutions. They are generally 

 disposed obliquely, ascending from below upward and 

 backward. Each convolution is about an inch" high, 

 rounded and slightly flattened on the summit. A verti- 

 cal section shows it to be narrower below, where it is 

 attached to the subcortical stem, broader and slightly 

 clubbed near the summit. Microscopically it is seen 



