THE NERVOUS SYSTEM 149 



synapses of neurons supported by branching neuroglia cells. 

 The cells of the grey matter are largest and most numerous 

 in the anterior 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 give off visceral 

 fibres (I.L.). In the dorsal region also a set of cells lie on 

 the mesial aspect of the posterior horn constituting the cells 

 of Lockhart Clarke (L.C.). 



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 is destroyed, the 

 animal lies for any length of time prone on its belly and 

 immovable. .If the skin of the leg is pinched the limb is 

 withdrawn, and if a piece of paper dipped in acetic acid is 

 placed on the flank, definite co-ordinated movements are 

 made to remove it. 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 spon- 

 taneous movements. 



The reflexes conriected with various groups of skeletal 

 muscles are definitely associated with different levels of the 

 cord. In man the reflex movement of the foot on tickling 

 the sole is connected with the part of the cord from which 

 the 1st, 2nd, and 3rd sacral nerves come off. The reflex of 

 the cremaster muscle with the 1st, 2nd, and 3rd lumbar; 

 that of the abdominal muscles with the 8th to the 12th 

 dorsal; of the epigastric muscles with the 4th to the 7th 

 dorsal ; and of the scapular muscles with the 5th cervical to 

 the 12th dorsal. By taking advantage of these reflexes the 

 condition of the cord at different levels may be studied. 



Reflex action in connection with various visceral muscles 

 are also connected with the spinal cord. Many of these are 

 complex reflexes involving inhibition of certain muscles and 

 increased action of others, some visceral, some skeletal. The 



