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COMPARATIVE ANATOMY 



The ventral corticospinal or direct pyramidal tract, formed of fibers 

 from the cerebral cortex of the same side which have synaptic connexions 

 with somatic motor cells. The size of this fasciculus is proportionate 

 to the use of the forelimbs and to the degree of intelligence. 



The rubrospinal tract, carries fibers from the red nucleus of the mid- 

 brain which cross the median plane and connect with somatic motor 

 cells in the ventral column of the spinal cord. 



Neuron Relations in the Cord. The spinal cord is both the center 

 for reflexes and the pathway for impulses towards and away from the 

 brain. 



In the simplest possible reflex action where two neurons only are 

 involved, the synaptic connexions between the two He in the gray matter 

 of the cord. The cell body of the afferent neurone is in the sensory 

 ganghon of the afferent nerve. That of the effector neuron is in the 

 ventral column of the spinal cord, and its neurite extends by way of the 

 ventral root to a muscle fiber or a gland. 



Usually, however, more than two neurons are chained together in a 

 reflex act, for there may be one or more association neurons located 

 in the gray matter of the cord, which carry the impulse from the receptor 

 neuron to the effector neuron. But it should not be understood that a 

 somatic motor nerve cell located in the ventral column has synaptic 

 connexions with the telodendria of only a single neurite. On the contrary, 

 many neurites may have synaptic relations with the dendrites of each 

 somatic motor neuron. The motor neuron is simply ''the final common 



path." 



Within the gray matter of the spinal cord, the central connexions 

 of the neurites of an afferent neuron may be of various kinds. The 

 telodendria may connect directly with the dendrites of a somatic motor 

 cell. This is the simplest relation. They may connect with a neuron 

 in Clark's column near the median line at the base of the dorsal column, 

 and the nervous impulse be carried in the dorsal cerebellar tract; or they 

 may pass to a commissural neuron and the impulse be carried to the 

 opposite side of the spinal cord. There are also connexions with sym- 

 pathetic neurons. 



The neurites of receptor neurons, having entered the spinal cord, 

 immediately dichotomize to give off long ascending and short descending 

 branches and thus add to the fibers of the fasciculus cuneatus. As these 

 fibers pass towards the brain, they are displaced inwards by the fibers 

 which are added from higher levels. The result of this process is that, 

 in the neck region, the neurites which enter the cord in the lower trunk 

 region come to lie in a median fasciculus, the fasciculus gracilis. Most of 

 the descending short fibers end in the gray matter of the cord. From 

 both ascending and descending neurites, fine collateral branches pass 



