FUNCTIONS OF THE SPINAL CORD. 795 



vaso-dilator centres. It thus appears that the calibre of the blood- 

 vessels in the body is regulated by a collection of centres located in the 

 central nervous system. 



Second, the cilio-spinal centre; this collection of nerve-cells is 

 located in the lower cervical portion of the cord ; its functions will be 

 again alluded to. The other centres, as of defamation, micturition, etc., 

 have been already mentioned. 



(b) The Spinal Cord as an Organ of Conduction. — It has been seen 

 in the consideration of reflex action that modes of communication exist 

 between the different nerve-cells in the spinal cord. It is also evident 

 that in the spinal cord must exist paths of conduction of sensorj* im- 

 pulses reaching the cord through the posterior roots of the spinal 

 nerves to the brain, and of the conduction of motor impulses from the 

 brain through the anterior spinal roots to the muscles. For when the 

 spinal cord is divided, or when it is altered by disease or injury, the 

 parts which receive their nerves from the portion of the spinal cord 

 situated below that part are paralyzed, both as regards sensation and 

 motion. Impressions made upon these parts are no longer appreciated 

 and voluntary movements can no longer occur in them, even although 

 reflex action may still be present. When the spinal cord is divided 

 above the points of origin of the nerves coming to the^muscles of respi- 

 ration, respiration is interfered with ; thus, if the spinal cord is divided 

 between the last cervical and the first dorsal vertebras, all the respiratory 

 muscles, with the exception of the diaphragm, are paralyzed. If the 

 spinal cord is divided above the origin of the phrenic nerve, then the 

 diaphragm is likewise paralyzed and death occurs from asphyxia. From 

 these facts it is evident that the spinal cord is the means of communica- 

 tion between the exterior and the brain, and we have now to consider 

 the paths which different impulses follow in passing from the centre to 

 the periphery and the reverse. 



In the first place, it would be scarcely conceivable that the irritation 

 from any localized portion of the skin surface could communicate 

 a definite localized sensation to the brain if the afferent impulse was 

 compelled to travel through the labyrinthine communications of the 

 gray cells of the spinal cord. Nor, again, could we suppose that 

 a single muscle could be thrown into contraction hy the will by passing 

 through the same complicated net-work of cells and fibres. It would be 

 much more natural to look for the paths of communication between the 

 voluntary muscles and the brain on the one side and the sensory end 

 organs and the brain on the other in the white columns of the cord. It 

 does not follow from this fact that the spinal cord may be regarded as a 

 bundle of white fibres connecting the periphery with the brain. Were 

 this the case we should expect that the spinal cord would increase in 



