SPINAL CORD. 109 



Decussation of Motor and Sensory Fibres. The Motor fibres, 

 which conduct volitional impulses from the brain outward to the anterior 

 cormice, arise in the motor centres of the cerebrum ; they then pass down- 

 ward through the corona radiata, the internal capsule, the inferior portions 

 of the crura cerebri, the pons Varolii, to the medulla oblongata, where the 

 motor tract of each side divides into two portions, viz. : I. The larger ; 

 containing 91 to 97 per cent, of the fibres, which decussates at the lower 

 border of the medulla and passes down in the lateral column of the oppo- 

 site side, and constitutes the crossed pyramidal tract. 2. The smaller, 

 containing 3 to 9 per cent, of the fibres, does not decussate, but passes down 

 the anterior column of the same side, and constitutes the direct pyramidal 

 tract, or the column of Ttirck. Some of the motor fibres of these two 

 tracts, after entering the anterior cornuae of the gray matter, become con- 

 nected with the large multipolar nerve cells, while others pass directly into 

 the anterior roots. Through this decussation each half of the brain governs 

 the muscular movements of the opposite side of the body. 



The Sensory fibres, which convey the impression made upon the peri- 

 phery to the cord and brain, pass into the cord through the posterior roots 

 of spinal nerves ; they then diverge and enter the gray matter at different 

 levels, and at once decussate, passing to the opposite side of the gray 

 matter. The sensory tract passes upward, through the cord, the medulla, 

 pons Varolii, the superior portion of the crura cerebri, the posterior third 

 of the internal capsule, to the sensory perceptive centre, located in the 

 hippocampus major and unciate convolution (Ferrier). Through this de- 

 cussation each half of the brain governs the sensibility of the opposite half 

 of the body. 



Properties of the Spinal Cord. Irritation applied directly to the 

 antero- lateral white columns produces muscular movements but no pain; 

 they are, therefore, excitable but insensible. 



The surface of the posterior columns is very sensitive to direct irritation, 

 especially near the origin of the posterior roots; less so toward the pos- 

 terior median fissure. The sensibility is due, however, not to its own 

 proper fibres, but to the fibres of the posterior roots which traverse it. 



Division of the antero-lateral columns abolishes all power of voluntary 

 movement in the lower extremities. 



Division of \ht posterior columns impairs the power of muscular coordi- 

 nation, such as is witnessed in locomotor ataxia. 



The gray matter is probably both insensible and inexcitable under the 

 influence of direct stimulation. 



A transverse section of one lateral half of the cord produces : 



