THE CEREBRO-SPINAL NERVOUS SYSTEM. 



483 



and special afferent fibres, even if few in number, exist, which are em- 

 ployed in the chain of such reflexes. 



T}. Motor Impressions. Motor impressions are conveyed down- 

 wards from the brain along the pyramidal tracts, viz., the direct or ante- 

 rior, 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 impressions 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 ante- 

 rior commissure. The motor fibres for the legs partially pass downwards 



FIG. 334. Diagram of the decussation of the conductors for voluntary movements, and those 

 for sensation: a r, anterior roots and their continuations in the spinal cord, and decussation at the 

 lower part of the medulla oblongata, mo; p r, the posterior roots and their continuation and 

 decussation in the spinal cord ; gr, gr, the ganglions of the roots. The arrows indicate the direction 

 of the nervous action; r, the right side; Z, the left side. 1, 2, 3, indicate places of alteration in a 

 lateral half of the spino-cerebral axis, to show the influence on the two kinds of conductors, result- 

 ing from section of the cord at any one of these three places. (After Brown-Sequard.) 



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- 

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

 fibres. It is therefore probable that each pyramidal fibre, or set of fibres, 



