THE SPINAL CORD. 387 



the cerebellum, or the "restiform bodies," are continuations from the 

 main part of the posterior columns of the cord. As these columns 

 diverge from each other at the medulla oblongata, leaving between 

 them the space of the fourth ventricle, they present the superficial 

 appearance of passing directly, on each side, from the cord to the cere- 

 bellum. But while all admit that a portion of each restiform body is 

 derived from the posterior column of the cord, observers differ as to 

 which portion is so derived ; and according to the views of Clarke and 

 Meynert,* the greater part undergo decussation in the interior of the 

 medulla oblongata, so that the restiform body of the right side is 

 formed of fibres from the left posterior column, and vice versa. 



The connections of the spinal cord with the brain, so far as they are 

 known with certainty, may be accordingly stated as follows : 



1. The greater part of the lateral columns, and a portion of the pos- 

 terior columns, after bilateral decussation, form the anterior pyramids, 

 which are continued in the superficial portion of the crura cerebri to 

 the corpora striata. 



2. The remainder of the lateral columns, together with the anterior 

 columns, pass by the deep-seated portion of the crura cerebri to the 

 optic thalami. 



3. The main portion of the posterior columns, perhaps after decus- 

 sation in the medulla oblongata, appear in the restiform bodies, and 

 thus reach the cerebellum. 



Transmission of Motor and Sensitive Impulses in the Spinal Cord and 



Nerves. 



The methods adopted for determining the functions of particular tracts 

 of the nervous system are twofold ; first, by applying an artificial stim- 

 ulus to the nerve or nervous tract, and observing the effect produced ; 

 secondly, by observing what nervous function is abolished when the 

 tract is divided or destroyed. In the peripheral nerves, which are 

 simply organs of transmission, both these methods yield definite results. 

 In the central parts, they are sometimes complicated by the mutual rela- 

 tions of the gray and white substances. 



Motor and Sensitive Transmission in the Spinal Nerves and Nerve 

 Hoots. If, in a living animal, a mechanical or galvanic stimulus be 

 applied to the anterior root of a spinal nerve, the effect of this irrita- 

 tion is a convulsive movement of the part to which the nerve is dis- 

 tributed. The muscular action is instantaneous, involuntary, and 

 m'omentary in duration ; and it is repeated with mechanical precision 

 each time the stimulus is applied. It is usually unaccompanied by any 

 indication of sensibility, and it is evidently a direct result of the excite- 

 ment of the anterior root. This root is therefore said to be " excitable," 

 because its irritation excites a movement in the corresponding parts. 



Furthermore, if the anterior root of a spinal nerve be divided, while 



*Huguenin. Anatomic des Centres Nerveux. Paris, 1879, p. 233. 



