542 TEXT-BOOK OF PHYSIOLOGY. 



filaments of medullated nerve-fibers coming from nerve-cells in lower 

 regions of the encephalo-spinal axis are also present, but for the most 

 part they pursue a tangential direction. 



2. The Layer of Small Pyramidal Cells. This layer consists mainly of 



nerve-cells, the majority of which are pyramidal in shape and of small 

 size. Other cells, however, are present, which present a variety of 

 shapes, for which reason the layer was at one time termed the ambiguous 

 layer. The apical process of the pyramidal cells is broad at the base, 

 but narrows rapidly as it passes upward. It frequently divides into sev- 

 eral branches, each of which develops club-shaped processes or gem- 

 mules, which give to it a feathery appearance. Dendrites are also given 

 off from the sides and base of the cell-body. From the base a single 

 axon descends which ultimately becomes the axis-cylinder of a med- 

 ulated nerve. 



3. The Layer of Large Pyramidal Cells. The nerve-cells of this layer, as the 



name implies, are also pyramidal in shape, but of large size. Each 

 cell presents the same features as the cells of the preceding layer, with 

 the exception that the apical process is larger, better developed, and 

 branches more freely. All the dendrites are extensively provided with 

 gemmules. The axon is well developed, sharply defined, and smooth. 

 After giving off collateral branches, the axon descends into the cere- 

 brum and becomes a medullated nerve-fiber. 



4. The Layer of Polymorphous Cells. In this layer the nerve-cells present a 



variety of forms: e.g., spindle, polygonal, pyramidal, etc. The spindle 

 form is the most common. From either end of the spindle a large 

 dendrite emerges, soon branches, and becomes gemmulated. The axon 

 is well defined and it soon descends into the white matter. 

 The Number of Cortical Cells. Attempts have been made by various 

 histologists to estimate the total number of functional nerve-cells in the cere- 

 bral cortex of man. Though the estimates are widely different, the lowest 

 presents numbers which are beyond comprehension. Thus, Meynert's 

 estimate is 612 millions; Donaldson's estimate for the entire brain is 12000 

 millions; and Thompson's 9283 millions. 



Structure of the White Matter. The white matter of the cerebrum 

 consists of medullated nerve-fibers which, though intricately arranged, may 

 be divided into three systems: viz., the commissural, the association, and the 

 projection. 



1. The commissural system. The fibers which compose this system unite 



corresponding areas of the cortex of each hemisphere. The fibers 

 from the frontal, parietal, and occipital lobes cross in the median line 

 and form a band of transversely arranged fibers, the corpus callosum. 

 The fibers which unite the corresponding areas of the temporo-sphe- 

 noidal lobes cross in the anterior commissure. All the commissural fib- 

 ers are the axons of nerve-cells in the cortex, the terminals of which are to 

 be found in the cortex of the opposite side. 



2. The association system. The fibers which compose this system unite 



neighboring as well as distant parts of the same hemisphere, and may 

 therefore be divided into long and short fibers. They associate the in- 

 excitable or association areas with the excitable or projection areas. 



