370 



CHORDATE ANATOMY 



cells of the motor nerves which supply visceral muscles, becomes more 

 distinctly demarked than in lower vertebrates. The gray matter crosses 

 the median plane of the cord as the gray commissure which surrounds 

 the central canal. 



A considerable increase in the amount of white matter in mammals 

 indicates a further dominance of the brain and a greater integration 

 of the body. The relative amount of white matter diminishes from the 

 medulla to the filum terminale in which the spinal cord ends. In the 

 region of the arms and legs, at the cervical and lumbar enlargements, both 

 gray and white matter increase in quantity. The division of the white 



PARIETAL LOBE 



INTERNAL CAPSULE 



CEREBRAL PEDUNCLE 



TEMPORAL LOBE 



CORTICAL MOTOR AREA 



CAUDATE NUCLEUS 



LENTICULAR NUCLEUS 



VEtTTRAL CEREBRO-SPIMAL TRACT 



U^TERAL CEREBRO-SPIMAL TRACT, 



(-VENTRAL ROOT OF SPINAL NERVE 



SPINAL CORD 



Fig. 328. — Diagram of the descending (pyramidal) conduction paths. (Redrawn after 



Morris.) 



matter into funiculi, begun in cyclostomes, reaches its climax in man. 

 Dorsal, lateral, and ventral funiculi are separated from one another not 

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

 and ventro-lateral 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 



