THE NERVOUS SYSTEM 



517 



only by dorsal and ventral nerve roots, but also by external dorso-lateral 

 and ventro-Iateral grooves or sulci, which extend lengthwise of the cord. 



The medullary sheaths of the fibers which compose the funiculi 

 develop at different times in ontogenesis as the fibers come into functional 

 activity. By the study of the time of myelination of fibers and their 

 degeneration after they are cut, it has been learned that fibers of similar 

 origin and function occur in bundles or tracts. Each funiculus consists of 

 a number of such tracts, together with groups of tracts or fasciculi. 

 A tract or a fasciculus may contain either ascending sensory fibers or 



PARIETAL LOBE-^>; 



INTERNAL CAPSULE 



CEREBRAL PEDUNCLE 



TEMPORAL LOBE 



CORTICAL MOTOR AREA 



CAUDATE NUCLEUS 



LENTICULAR NUCLEUS 



VENTRAL CEREBRO-SPINAL TRACT 



LATERAL CEREBRO-SPINAL TRACT. 



't-VENTRAL ROOT OF SPINAL NERVE 



SPINAL CORD 



Fig. 431. — Diagram of the descending (pyramidal) conduction paths. (Redrawn after 



Morris.) 



descending motor fibers. A fasciculus may contain both kinds. Fibers 

 may have their origin or termination at any level and relatively few 

 extend the entire length of the cord. (Fig. 430) 



Among the more important ascending fiber tracts are: 

 The fasciculi gracilis and cuneatus, which consist chiefly of sensory 

 fibers from the dorsal roots. Most of these sensory neurites connect 

 with neurones in the spinal cord, chiefly with association neurones of 

 the Golgi type II. Those which reach the brain have synaptic relations 

 with secondary neurones in the wall of the medulla oblongata. 



The fasciculus spinocerebellaris dorsalis, formed by neurites of 

 ganglion cells in the nucleus dorsalis or Clark's column of the cord. These 

 fibers reach the cerebellum by way of the restiform body. 



