490 A MANUAL OF VETERINARY PHYSIOLOGY 



tract for voluntary body movement which we have studied at 

 p. 465 under the name of the ' crossed pyramidal tract.' It is the 

 path by which movements of which the animal is distinctly 

 conscious are effected. The brain starts or instructs the move- 

 ment, the mechanisms in the cord carry it out. This places 

 quadrupeds in much the same position as the decerebrated frog. 

 The apparently purposeful movements executed by the cord of 

 the animal immediately after death warrants this supposition. 

 The limbs are represented in the cortical area of the fissure of 

 Rolando, and they are also represented in the grey matter of the 

 spinal cord, the great difference between the two centres being 

 that one can be exercised voluntarily, while the other is in- 

 voluntary and unconscious, until the moment arrives when the 

 pulling of the needful switch brings it under the control of the 

 higher centres. 



We shall see presently that the entire cerebral cortex may be 

 removed from the frog, pigeon, and dog, and the motor area 

 from the sheep, without producing in these mammals more than 

 a marked loss of power, which is regained in time. In such cases 

 the centres in the cerebrum controlling the muscles are entirely 

 cut off from them, and other tracts have to be found. In the 

 dog it is suggested that the rubro-spinal tract (p. 466) fulfils this 

 function. In the sheep we have seen (p. 466) that motor fibres 

 are only in part derived from the cerebral cortex, and that a 

 sub-cortical supply is furnished by the mid -brain, pons, and 

 medulla. There can be little doubt that a similar arrangement 

 exists in the other herbivora. 



The double neurone relay between cortex and end plate is the 

 explanation of a peculiarity observed in muscular paralysis in 

 man. When the neurone between the cord and the muscle is 

 affected, the paralysis is complete; but when the pyramidal 

 neurones — viz., those between the cortex and the grey matter 

 of the cord — are affected, there is a path left open for reflex 

 stimulation, and those impulses normally passing to muscles 

 from the cerebellum, which have been spoken of as 'tonic,' 

 throw the paralysed limbs into a condition of continuous con- 

 traction. This is spoken of as spastic paralysis. 



The Great Afferent or Sensory Path conveys impulses evoking 

 sensations — heat, cold, touch, and pain — also such senses as are 

 not normally recognised — viz., muscle sense, joint, tendon, and 

 viscera sense, and the senses of vision, hearing, smell, and taste. 

 This path is broken by at least three sets of synapses : (1) in the 

 cord ; (2) in optic thalamus ; and (3) in cortex cerebri. The optic 

 thalamus is practically the meeting-place or junction of the 

 whole afferent system on its way to the cerebral cortex (Fig. 141). 



Impulses conveyed by sensory nerves pass through the spinal 



