THE CENTRAL NERVOUS SYSTEM. 



which diverge from the chief stem as the primary and secondary 

 medullary branches ; these form the ' ' arbor vitae. ' ' 



The blood-vessels supplying the cerebellum, principally branches 

 of the vertebral and basilar arteries, after repeated division within 

 the pia, send small branches vertically into the molecular layer 

 as far as its inner boundary ; a rich vascular net-work surrounds 

 the cells of Purkinje : while capillaries are wanting within the pe- 

 ripheral zone of the molecular layer, they are well represented 

 in the granule layer and among the nerve-bundles of the me- 

 dulla, where the blood-vessels run between the fibres and form elon- 

 gated meshes which correspond to the disposition of their tracts. 



THE CEREBRUM. 



The cerebral hemispheres consist of a thin outer sheet of gray 

 matter, the cortex, which everywhere covers in the white matter of 

 the medulla, accurately following the intricacies of the convoluted 

 surface of the brain ; in addition to the cortex, large special masses 

 of gray matter lie within the medullary substance and take part in 

 the constitution of the nucleus caudatus, the nucleus lenticu- 

 laris, the thalamus, the corpus subthalamicum, and the minor 

 collections of lesser importance. 



The cerebral cortex forms a dark, peripheral zone, 2-4 mm. in 

 thickness, which is best developed in the ascending frontal and the 

 paracentral convolution, being thicker at the summit of the gyri than 

 in the fissures ; the gray stratum appears least conspicuous in the 

 posterior part of the occipital lobe. 



The arrangement of the elements of the cortex in layers is indi- 

 cated by the stratification which the vertically-cut surface of the 

 cortex presents even to the unaided eye ; in favorable situations 

 three bands are distinguishable, an outer white, a middle gray, 

 and an inner yellowish red ; in certain regions, as the superior 

 frontal, the precentral, and the occipital convolutions, the layers are 

 increased to six by the addition of the stripes of Baillarger. These 

 markings, however, do not accurately represent the structure of 

 the cortex, which can be studied adequately only in successfully- 

 stained sections cut vertically to the free surface of the convolution 

 and parallel with the general course of the nerve-fibres. In such 

 preparations five zones are recognizable, which, however, are not 

 sharply defined from one another, but are often blended. 



i. The first or outer layer, next the pial surface, about .25 mm. 

 in thickness, is composed essentially of neuroglia, together with 

 numberless delicate terminal ramifications of the protoplasmic 

 processes of nerve-cells situated within the deeper layers, and a 

 few tangential nerve-fibres ; the protoplasmic threads contributed 



