FINAL REVIEW. 407 



from the cortex of the cerebrum the converging pyramidal fibers, as the second 

 division of the chain: nerve^ nucleus — pyramidal tract, cortex. 



As long as the first division remains intact, the corresponding muscles may be 

 made to contract by electric, mechanical, and reflex irritation in animals, even also 

 to a certain degree by voluntary impulse; if nerve or nucleus be destroyed, how- 

 ever, positive paralysis results. 



To a complete voluntary ability it is necessary that both divisions be sound; 

 indeed, with the highly developed brain of man when the second division is inter- 

 rupted there is no movement possible as a result of the will. When one suffers a 

 stroke of apoplexy with tearing of the internal capsule, the muscles of the opposite 

 side of the body are not really paralyzed; they can no longer be brought into con- 

 traction by the will, but may by other means. It is different when in infantile 

 spinal paralysis, for example, a nucletis itself is destroyed; then follows a real 

 paralysis, which, generally irreparable, leads to atrophy, and reacts but little to 

 reflex or other stimulation. It makes a great difference as to the prospect of the 

 recovery of function whether the cerebral tract be interrupted or some place lower 

 down. 



5. The coronal fibers to the pons arise from the cortex cerebri, espe- 

 cially the temporo-oceipital lobe; perhaps also from the frontal lobe. They 

 extend through the internal capsule to the crusta of the pes and then to 

 the pons. Experiments made by the degeneration method show that they 

 extend no farther than to groiips of ganglion-cells there found. But to 

 these same groups one can trace fibers coming from the opposite half of the 

 cerebellum (cerebellar peduncles). 



6. The sensory nerves arise from cells in the spinal ganglia. At least in 

 vertebrates no other origin has been demonstrated for them. Since we know, 

 however, that the optic and olfactory nerves contain fibers which, arising 

 from sense-epithelium, pass centrally; since, further, it has been shown 

 in invertebrates that there are fibers arising out of epidermal sense-epithe- 

 lium, it were not impossible that there should be fibers in sensory nerves 

 which originated in the periphery. The process of secondary degeneration 

 after section of the nerve speaks against this supposition. 



From the cells of the spinal ganglia arises a second tract: the spinal 

 root, which enters into the cord. A part of the root splits up in the pos- 

 terior horns, or, it may be, in the nuclei of the cranial sensory nerves, arbor- 

 izing around cells (distal nuclei of the sensory nerves). 



Another portion first courses a distance in the central organ, either 

 upward, as the spinal nerve-fibers of the posterior columns, or downward, as 

 many roots of the cranial sensory nerves, before it ends in distal nuclei. 



From the cells of these niiclei arises the central sensory tract, or the tract 

 of second ord^. It arrives either at the level of the entrance of the root 

 into the central axis, or higher up, always in the territory of the decussated 

 lemniscus. But it extends toward the cerebruin with the lemniscus. 



7. "We do not yet know the cells of origin nor the terminations of 



