MEDULLA AND BASAL GANGLIA. 529 



SUMMARY OF THE STRUCTURE OF THE MEDULLA, ISTHMUS, AND 



BASAL GANGLIA. 



Structure of the Central Gray Matter. Though the general arrange- 

 ment of the central gray matter has been incidentally alluded to in the fore- 

 going presentation of the anatomic features of the medulla and isthmus, it 

 will be convenient to summarize its arrangement and structure at this point. 



The gray matter of the cord, of the dorsal aspect of the medulla and pons, 

 of the region surrounding the aqueduct of Sylvius, and of the lining of the 

 third ventricle, constitute practically a continuous system, though presenting 

 modifications in various parts of its extent. In the transition region of the 

 spinal cord and medulla the gray matter of the former becomes much changed 

 in shape owing to the shifting of position of the various tracts of white matter, 

 until in the medulla 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 dendrites "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 situ- 

 ated 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 merge 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 found in the central gray matter may be divided into two 

 groups efferent and afferent. 



The efferent cells are motor in function, inasmuch as the excitation 

 arising in them is transmitted outward through their related axons to, and 

 exciting movement in, skeletal muscles, glands, viscera or blood-vessels. 



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 composed 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 through the crus 

 cerebri to the pons, medulla, and spinal cord. They may be divided into 



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