742 COURSE OF THE MOTOR AND SENSORY TRACTS. 



in turn covers the layer of irregularly formed ganglion-cells. Each ganglion-cell 

 possesses a neurite and numerous dendrites. Between the cells everywhere lie 

 large numbers of bundles of medullated nerve-fibers. 



The cortex of the brain consists of peripheral gray matter with numerous 

 convolutions and sulci (Fig. 254, C). This can be recognized as the central organ 

 of the nervous system from the presence of large numbers of ganglion-cells. From 

 it pass all of the motor fibers that can be stimulated by the mind (will, conception) , 

 and to it pass all of the fibers derived from the organs of special sense and the 

 sensory organs that intermediate the psychical perception of external impressions. 



All of these tracts together, in part corticopetal, in part corticofugal, pursue 

 in general a convergent course toward the central part of each cerebral hemisphere, 

 where the large central ganglia of the brain are situated, corpus striatum (C. s.), 

 lenticular nucleus (N. 1.), optic thalamus (T. o.) and quadrigeminate bodies (v). 

 Some fibers merely pass by these structures (5, 5), but many enter the central 

 gray matter. The fiber-system mentioned, which has a radiating arrangement 

 within the cerebral hemisphere, is known as the corona radiata, or the projection- 

 system of the first order. In addition to this, the white matter contains two other 

 groups of fibers, namely (a) the commissural fibers corpus callosum and anterior 

 commissure (c c) , which connect the two hemispheres ; and (6) the association- 

 fibers, by means of which different areas of the cortex of the same side are con- 

 nected (a a) . 



The large, cellular, gray masses of the central cerebral ganglia form the first 

 stage in the course of a large number of fibers of the projection-system of the 

 first order. On entering these central masses the fibers undergo an interruption 

 in their course, while a reduction in the number of fibers of the corona radiata 

 takes place. In detail the relation of the fibers of the corona radiata to the great 

 central ganglia according to Meynert is as follows: The entire mass of fibers of 

 the system of the corona radiata breaks up in general into as many bundles as 

 there are ganglia on each side. There are thus systems for the striate body (i, i), 

 the lenticular nucleus (2, 2), the optic thalamus (3, 3) and the quadrigeminate 

 bodies (4, 4). 



According to Flechsig the convolutions of the brain can be divided 

 into two groups: The first group contains only association and com- 

 missural fibers, by means of which the convolutions are connected with 

 other cortical areas. The other group contains in addition bundles 

 which pass to the optic thalamus in the corona-radiata. These Flechsig 

 designates sense-centers, of which' each possesses its own motor ap- 

 paratus. 



The central convolutions, the seat of the cutaneous and the muscular sense, 

 serve as the principal source of origin for the association-tracts. Then the auditory 

 area, and in lesser degree the visual area, play an important role as the source 

 of origin of the association- tracts. 



From the large central ganglia there develops, later passing downward, 



the projection-system of the second order, whose longitudinal fibers reach their 



temporary termination in the so-called gray matter of the central cavity. This 



is the cellular gray matter that extends from the third ventricle through the 



aqueduct of Sylvius, the floor of the fourth ventricle, to the lowermost portion 



the gray matter of the spinal cord, occupying the interior of the medul- 



A ry tl i- 6 * represents likewise the second stage in the course of the fibers. 



Accordingly, the projection-system of the second order extends from the large 



tral ganglia of the cerebrum downward to the gray matter of the central 



cavity. The fibers of this system must obviously be of widely varying length, 



some terminating in the gray matter of the central cavity above the medulla 



ongata (oculomotor origin) , while others extend to the level of the last spinal 



'TIT 6 ' * J gra ^ matter of the central cavity forms a mass for the interruption 



the fibers, and an increase in the number of fibers takes place in it, for many 



lore fibers pass out from the gray matter of the medulla oblongata and the spinal 



cord to the periphery than were sent to it from above from the central ganglia 



of the cerebrum. 



With reference especially to the arrangement of the fibers of this projection - 



the second order, it is assumed that the fibers descending from the 



:icular nucleus and the striate body (8, 8) unite to form a special tract passing 



