466 A MANUAL OF VETERINARY PHYSIOLOGY 



the proportion the crossed pyramidal system bears in other 

 animals : 



Cat 776 



Rabbit - 5-30 



Guinea-pig - - - - - - -3-00 



Mouse - - - - ^ - - - i-oo 



Frog - Absent 



The crossed pyramidal tract is so named on account of its fibres 

 crossing in the medulla on their way to the cord. In the direct 

 pyramidal tract there is no crossing. The effect of the fibres 

 crossing is that the right cerebrum controls the muscles on the 

 left side of the body, and vice versa. If an animal be shot in the 

 right brain it falls on the left side, as these muscles are the first 

 to be paralysed. 



In the dog, and perhaps in other of the lower animals, there 

 are other paths than the pyramidal conveying impulses which 

 lead to motion, for if all the pyramidal fibres be divided, com- 

 plete paralysis does not follow, while stimulation of the motor 

 areas of the cerebrum continues to produce muscular con- 

 traction. It is believed that the supplemental path is the 

 rubrospinal tract, which takes its origin in the red nucleus of 

 the mid-brain, and is considered by Dexler* to replace in animals 

 the direct pyramidal tract. 



Recently J. L. King has investigated the crossed pyramidal tract 

 of the sheep.f He finds it to be imperfectly developed, and that it 

 does not extend beyond the first cervical segment. In the medulla 

 some of its fibres decussate, others are uncrossed. This short tract 

 represents all the motor fibres having origin in the cortex of the 

 cerebrum ; from this we should expect that not only are the cortical 

 motor areas small, but that motor fibres must be derived from some 

 other portion of the brain. King has found this to be the case. In 

 the ventro-lateral columns of the cord of the sheep are two well- 

 developed descending paths running its entire length. One of these 

 he identifies as the rubro-spinal tract. These two tracts represent 

 in the sheep the chief primary motor paths in the cord. Centrally 

 they are connected with the mid -brain, pons, and medulla. The fibres 

 are consequently sub-cortical in origin. The two motor paths are 

 reinforced by proprio-spinal fibres, which brings them into intimate 

 relation with near and distant segments of the cord. King concludes 

 that the chief motor paths in the spinal cord of lower mammals do 

 not, as in man, originate in the central cortex, but from a point 

 lower in the brain. 



Rubrospinal Tract. — Known also as the Ventro- Lateral 

 Descending Tract, lies just below the crossed pyramidal tract 

 (Fig. 139, A). "] It arises in the mid-brain from the red nucleus. 



* ' Veterinary Anatomy,' S. Sisson. 



| The Quarterly Journal of Experimental Physiology, vol. iv., No. 2, 

 June, 191 1. 



