MEDULLA AND BASAL GANGLIA. 477 



and pons it is spread out in the form of a thin layer near their dorsal 

 surfaces, where, together with the ependyma, it forms the floor of 

 the fourth ventricle. 



In the region of the aqueduct of Sylvius the gray matter again 

 converges and ultimately surrounds the canal, to again expand at its 

 anterior extremity, to form the lining of the third ventricle. 



The Nerve-cells. The nerve-cells in these different regions do 

 not differ morphologically from those in the gray matter of the spinal 

 cord. The corpus, or body of the cell, presents a number of den- 

 drites as well as the sharply defined axon. As a rule, the cells are 

 arranged in groups, or clusters, or nests, partially surrounded and 

 enclosed by supporting tissue, and situated beneath the floor of the 

 fourth ventricle and the floor of the aqueduct of Sylvius. From some 

 of the cell groups axons pass ventrally through the white matter to 

 emerge on the ventral and lateral surfaces of the medulla, pons, and 

 crura, where they are known as efferent or motor cranial nerves. 

 From other groups of cells, axons cross the median line, and after 

 joining the mesial fillet ascend toward the cerebrum. Around these 

 latter cells the terminal filaments of the afferent or sensor cranial 

 nerves arborize. The collection of cells formed in the central gray 

 matter may be divided into two groups efferent and afferent. 



The efferent cells, like those of the cord independent of a trophic 

 influence, are motor in function, inasmuch as the excitation arising 

 in them is transmitted outward through their related axons to mus- 

 cles, glands, or blood-vessels, imparting to them motion, either 

 molar or molecular. 



The afferent cells are largely sentient or receptive in function, 

 inasmuch as the excitations brought to them by the afferent cranial 

 nerves from skin and mucous membranes and from sense-organs, 

 such as the tongue and ear, are received by them and transmitted 

 through their ascending axons to the cortex of the cerebrum, where 

 they are translated into conscious sensations. 



Structure of the White Matter. The white matter is com- 

 posed of medullated nerve-fibers, and though arranged in a very 

 complex manner may be divided into longitudinal and transverse 

 fibers. 



The longitudinal fibers which compose the main portion of the 

 isthmus may be subdivided into (i) a ventral or pedal portion and (2) 

 a dorsal or tegmental portion. The fibers constituting the ventral 

 or pedal portion may for convenience be said to extend from the 

 cerebral cortex to the pons, medulla, and spinal cord. They may be 

 divided into three distinct tracts: e. g. y the pyramidal tract, the 

 fronto-cerebellar tract, and the occipito-temporo-cerebellar tract 

 (Fig. 217). 



The pyramidal tract descends from the cortex of the cerebrum 



