50 ESSENTIALS OF PHYSIOLOGY. 



column, there are four main groups of collaterals, including the termina- 

 tions of main fibres, which run into the grey matter. These are (1) 

 fibres to the anterior horn of the grey matter, (2) fibres to the posterior 

 horn, (3) fibres to Clarke's column, and (4) fibres to the grey matter 

 of the opposite half of the spinal cord, running across in the grey 

 commissure. All these fibres end by forming arborisations in relation 

 with nerve cells. 



THE FUNCTIONS OF THE SPINAL COED. 



The functions of the spinal cord are two : (1) it is a centre or series 

 of centres for reflex actions, and (2) it conducts impulses between the 

 higher centres in the brain and the spinal nerves which transmit these 

 impulses to or from the active or receptive tissues of the body. It is 

 possible that the spinal cord may possess a low degree of automatic 

 activity under certain conditions. Graham Brown has recently shown 

 that in an animal from which the entire brain has been removed, and 

 in which the afferent nerves from the limbs have been divided, there 

 may occur under a certain degree of anaesthesia alternate flexion and 

 extension of the limbs. No definite conclusions can, however, be based 

 at present on this experiment. 



SECTION III. 

 CONDUCTION IN THE SPINAL CORD. 



The spinal cord forms a pathway for impulses which originate in the 

 brain, and are distributed to all parts of the body through the anterior 

 spinal nerve roots and the spinal nerves. It also conducts impulses 

 which are set up by stimulation of afferent nerve endings, chiefly in the 

 skin and muscles, and which reach it by the spinal nerves and posterior 

 spinal nerve roots to pass in an upward direction to the brain. 



There are two methods of conduction : (1) by long tracts of nerve 

 fibres situated in the white matter, and (2) by short association tracts 

 or relays. Most of the long tracts are found in the peripheral part of 

 the white matter. The short tracts lie more deeply, their fibres run- 

 ning a little way in the white matter, then turning into the grey matter, 

 to terminate by arborisations in relation with nerve cells, the axons of 

 which in their turn form other short tracts in the white matter ; so that 

 impulses are conducted by a series of relays. 



The tracts have been mapped out by two methods : (1) by observing 

 the time during development at which the fibres in the various 

 areas of the white matter of the spinal cord acquire a myelin sheath, 



