122 HISTOLOGY. 



(2) cells with processes limited to the central nervous system and extending 

 through its white substance from one part to another; and (3) cells with 

 processes limited to the gray substance. 



The fibers of the central nervous system are the processes of these 

 three types of cells together with those which enter from the peripheral 

 ganglia. These fibers are arranged in bundles or fasciculi as they traverse 

 the white substance. The boundaries of the bundles are not indicated in 

 ordinary sections and are never sharply outlined. They have been deter- 

 mined in various ways, such as cutting certain parts of the cord and 

 observing in sections the path of the fibers which degenerate and lose 

 their myelin in consequence. These results are confirmed by the exami- 

 nation of embryos in which certain fiber tracts develop their myelin 

 sheaths earlier than others. It has been found that each dorsal funiculus 

 includes two large fasciculi, the cuneate and gracile, respectively. The 

 cuneate fasciculus which is the more lateral, receives the fibers of the 

 dorsal root. In it they divide into ascending and descending fibers and give 

 off the reflex collaterals to the motor cells as shown in the diagram 

 (Fig. 147). The ascending fibers in their course up the cord to the brain 

 approach the median septum thus entering the gracile fasciculus. The 

 manner in which they communicate with the cells of cerebral hemi- 

 spheres will be considered with the brain. 



The lateral funiculus of the cord consists of four fasciculi, (i) The 

 cerebellospinal fasciculus consists largely of fibers from the dorsal nucleus 

 ascending to the cerebellum. (2) The superficial ventro-lateral fasciculus 

 also contains fibers ascending to the cerebellum. Descending fibers from 

 the cerebellum, together with large numbers of those connecting the 

 different levels of the cord with one another, are found in the lateral fas- 

 ciculus (3). (4) The lateral cerebrospinal fasciculus is the descending tract 

 from the cerebral hemispheres to the motor cells, being the path of volun- 

 tary motor action. These tracts cross in the brain so that the right tract 

 of the cord is connected with the left hemisphere and vice versa. 



The ventral funiculus includes two fasciculi. The ventral fasciculus 

 consists chiefly of fibers connecting the lateral halves of the cord and its 

 different levels with one another. The small ventral cerebrospinal fascic- 

 ulus contains descending fibers from the hemispheres, most ,of which cross 

 through the white commissure to connect with motor cells on the opposite 

 side of the cord. Some, like the fiber shown in the figure, may have crossed 

 at a higher level in the cord. Such fibers as cross in the cord are believed 

 not to cross in the brain so that all the motor cells are thus in communication 

 with the opposite hemispheres of the brain. 



The ependyma is that part of the neuroglia which lines the central 



