122 HUMAN PHYSIOLOGY. 



FUNCTIONS OF THE SPINAL CORD, 



1. As a Conductor. The Lateral columns, particularly the posterior 

 portions, the " pyramidal tracts," and the columns of Tiirck, are the chan- 

 nels through which pass the voluntary motor impulses from the brain to the 

 large multipolar nerve cells in the anterior cornuae of gray matter, and 

 through them become connected with the anterior roots which transmit the 

 motor stimuli to the muscles. 



The Anterior columns, especially the portion surrounding the anterior 

 cornuae, the " anterior radicular zones," are composed of short longitudinal 

 commissural fibres, which serve to connect together different, segments of 

 the spinal cord, a condition required for the coordination of muscular 

 movements. 



The Posterior columns are composed of short and long commissural 

 fibres which connect together different segments of the cord. They are 

 insensible to direct irritation, but aid in the coordination of muscular move- 

 ments in walking, standing, running, etc. Degeneration of the posterior 

 columns gives rise to the lack of muscular coordination observed in loco- 

 motor ataxia. 



The Gray matter, and especially that portion immediately surrounding 

 the central canal, transmits the sensory nerve fibres from the posterior roots 

 up to the brain. Decussation of the sensory fibres takes place throughout 

 the whole length of the gray matter. 



The Multipolar cells of the anterior cornua are connected with the 

 generation and transmission of motor impulses outward; are centres for 

 reflex movements; are the trophic centres for the motor nerves and muscu- 

 lar fibres to which they are distributed. The anterior roots give passage 

 to the vaso-constrictor and vaso dilator fibres which exert an influence 

 upon the calibre of the blood vessels. Complete destruction of the anterior 

 horns is followed by a paralysis of motion, degeneration of the anterior 

 roots, atrophy of muscles and bones, and an abolition of reflex move- 

 ments. 



2. As an Independent Nerve Centre. 



The spinal cord, by virtue of its contaiaing ganglionic nerve matter, is 

 capable of transforming impressions made upon the centripetal nerves into 

 motor impulses, which are reflected outward through centrifugal nerves to 

 muscles, producing movements. These reflex movements taking place 

 through the gray matter, are independent of sensation and volition. 



The mechanism involved in every reflex act is a sentient surface, a sensory 

 nerve, a nerve centre, a motor nerve and muscle. 



