158 VETERINARY PHYSIOLOGY 



the ventral arid dorsal horns (A. and P.) which divide the 

 white matter surrounding the grey into a dorsal, a lateral, 

 and a ventral column. In the dorsal region a lateral horn 

 of grey matter projects into the lateral column (I.L.). The 

 grey matter on each side is joined to that of the opposite 

 side by bands of grey matter ; the ventral and dorsal grey 

 commissures, one below and one above the central canal. 



The grey matter is composed very largely of cells and 

 synapses of neurons supported by branching neuroglia cells. 

 The cells of the grey matter are largest and most numerous 

 in the ventral horn, where they constitute the cells from 

 which the majority of nerve fibres come off. In the dorsal 

 region a group of cells in the lateral horn, the intermedio- 

 lateral cells, give off visceral fibres (I.L.). In the dorsal 

 region also a set of cells lie on the mesial aspect of the 

 dorsal horn constituting the cells of Lockhart Clarke (L.C.). 



The white substance is composed of medullated nerve fibres 

 in which the neurilemmal sheath is absent. The fibres chiefly 

 course up and down the cord, and some run in a horizontal 

 direction : 1. The fibres of the spinal nerves ; 2. Fibres pass- 

 ing from grey to white matter ; 3. Fibres joining the two sides 

 of the cord in front of the ventral grey commissure forming 

 the white commissure. 



Functions 



The spinal cord is the great mechanism of reflex action, 

 and the great channel of conduction between the brain and 

 the peripheral structures. 



A. REFLEX FUNCTIONS 



If the brain of such an animal as a frog be destroyed, the 

 animal lies prone on its belly and immovable for any length 

 of time ; but the legs tend to be drawn up alongside the body, 

 and the muscles are in a state of slight tonic contraction 

 very different from the flaccid condition found after destruc- 

 tion of the cord. The study of the spinal reflexes (p. 90) has 

 shown that the animal has the power of reflex movements with 

 definite co-ordination of its muscles, but it has no power of 

 balancing itself, and manifests no spontaneous movements. 



