574 TEXT-BOOK OF PHYSIOLOGY 



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. 



Classification of Nerve-cells. The cells of the gray matter may be 

 divided into three main groups; viz., intrinsic or associative, receptive or 

 afferent and emissive or efferent. 



The intrinsic cells are associative in function. The axons to which these 

 cells give origin pass more or less horizontally into the white matter, where 

 they divide into two branches, one of which passes upward, the other down- 

 ward. At various levels they re-enter the gray matter and arborize around 

 other intrinsic cells. 



The receptive cells are largely sentient or afferent in function, inasmuch as 

 they receive the nerve impulses transmitted to them by afferent cranial 

 nerves. As the afferent nerve fibers are classified, in accordance with the sensa- 

 tions to which they give rise, as sensor, thermal, tactile, gustatory, auditory, 

 etc., so these cells may be similarly classified, according as they transmit their 

 excitations to those specialized areas in the cerebral cortex in which these 

 different sensations arise. 



The emissive cells are efferent or motor in function, inasmuch as the 

 excitation arising in them is transmitted outwardly through their axons to 

 muscles, glands, blood-vessels, and viscera, imparting to them motion, 

 either molar or molecular. As the efferent fibers are classified (see page 99) 

 in accordance with their physiologic action into motor, vaso-motor, secretor, 

 viscero-motor, so the nerve-cells of which the nerves are integral parts may 

 be classified physiologically as motor, vaso-motor, secretor, and viscero- 

 motor. Collections or groups of such cells are termed "centers." 



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 

 three distinct tracts: e.g., the pyramidal tract, the fronto-cerebellar tract, 

 and the occipito-temporo-cerebellar tract (Fig. 239). 



The pyramidal or motor tract descends from the cortex of the cerebrum 

 mainly from the gyrus anterior to the central fissure, passes through the 

 posterior one-third of the anterior segment and the anterior two-thirds of the 

 posterior segment of the internal capsule, the middle two-fifths of the crusta, 



