574 HANDBOOK OF PHYSIOLOGY. 



these sensations of temperature, pain, and touch. If one set of fibres is 

 destroyed by disease, others seem able, through the collaterals, to take 

 up its function. We can only say that most of these sensory impressions 

 pass up in the lateral and anterior columns. It is probable, also, that 

 pain and temperature sensations cross over at once, to a considerable ex- 

 tent, and pass up in the opposite side of the cord to which they enter. 

 Touch and pressure sensations, as well as muscle-sense impressions, and 

 sensations of equilibrium, pass up largely upon the same side until they 

 reach the medulla or cerebellum. 



The direct cerebellar tract is believed to commence in the cells of the 

 posterior vesicular column of Clarke of the same side; it goes chiefly to 

 the cerebellum, through the restiform body, but is said also to contain 

 fibres which pass up as far as the corpora quadrigemina and then turn 

 backward and lying near the brachium pass to the cerebellum. The 

 fibres of the antero-lateral ascending tract are believed to arise from 

 the gray matter of the posterior cornu. In the case of the ascending 

 tracts, with the exception of the posterior median column, the connec- 

 tion with the posterior nerve-roots is not direct. 



b. Motor Impressions. Motor impressions are conveyed down- 

 ward from the brain along the pyramidal tracts, viz., the direct or an- 

 terior, and the crossed or lateral, chiefly in the latter. Generally 

 speaking, the impressions pass down on the side opposite to which they 

 originate, having undergone decussation in the medulla; but some im- 

 pressions do not cross in the medulla, but lower down, in the cord, being 

 conveyed by the anterior or uncrossed pyramidal fibres, and decussate in 

 the anterior commissure. The motor-fibres for the legs partially pass 

 downward in the lateral columns of the same side. This is also probably 

 the case with the bilateral muscles, i.e., muscles of the two sides acting 

 together, such as the intercostal muscles and other muscles of the trunk, 

 as well as the costo-humeral muscles. 



It is quite certain, as was just now pointed out, that the fibres of the 

 anterior nerve-roots are more numerous than the fibres proceeding down- 

 ward from the brain in the pyramidal tracts, or the so-called pyramidal 

 fibres. This is because each pyramidal fibre is really a very long nerve 

 process or neuraxon, and is supplied in its course with a large number 

 of collaterals, which gooff at different points, and thus put it in relation 

 with different groups of nerve-cells in the anterior cornua at various 

 levels. Each nerve-fibre of the pyramidal tract, by means of its col- 

 laterals, can control a number of nerve-cells, and can thus co-ordinate 

 the action of impulses sent out through the anterior roots to a number 

 of groups of muscles. In other words, the gray matter of the anterior 

 cornua contains an apparatus with various complicated co-ordinating 

 powers, which apparatus is under the control of the neurons whose 

 cells of origin are in the cortex of the brain. This apparatus is also re- 

 flexly influenced by sensory impressions passing to the cord. 



