198 EQUINE PHYSIOLOGY. 



The sensory nerves decussate all along the cord, not at one fixed 

 point, ending at the sensory portion of the internal capsule and 

 hippocampus major. Thus one side of the brain governs the 

 sensation of the opposite side. 

 Functions of the Cord. 



I. As a conductor. II. As an independent nerve centre. 



I. As a Conductor. The inferior and inferior lateral columns 

 convey motor impulses from the brain to the periphery. If a sec- 

 tion of both motor tracts is made, motor paralysis will result below 

 that point. If one tract, motor paralysis on the same side only. 



If the sensory portion of the cord is cut completely, a similar 

 absence of sensation below that point and inability to coordinate 

 will be the result. If a unilateral sensory section is made, there 

 will be complete loss of sensation on the opposite side. 



Section of one lateral half of the cord causes motor paralysis 

 on the same side, with hyperassthesia and sensory paralysis on the 

 other. 



II. As a Nerve Centre. If a frog is decapitated and placed in 

 a vacuum it will remain quiet. Now, if its surface is irritated, mus- 

 cular contraction will result. If the cord is broken up no such 

 phenomena take place, showing that the cord controlled these 

 movements, which are called reflex actions. The sensory nerves 

 convey the impression to a centre in the cord, which sends it 

 back to the muscles, causing a contraction (Flint). 



The gray substance of the cord is neither sensible nor excitable. 



The inferior lateral white columns are excitable throughout, but 

 not sensible. 



The superior columns are sensible, more so at the entrance of 

 the posterior nerve-roots. They are not excitable. Deeply, they 

 are neither excitable nor sensible. 



THE ENCEPHALON. 



The encephalon consists of the medulla oblongata, pons, corpora 

 striata, corpora quadrigemina, optic thalami, olfactory lobes, cere- 

 bellum and cerebrum. 



MEDULLA OBLONGATA. 



This serves as a medium of transmission of motor and sensory 

 impulses, and also as the centre for numerous functions, as masti- 

 cation, deglutition, vomiting, a cardiac centre, a vasomotor centre 



