78 ANATOMY OF THE RABBIT 



coming stimuli, without conscious experience being a necessary 

 factor in producing the result. In vertebrates in which the spinal 

 cord is divided, the lower part thus being separated from the brain, 

 stimulation of the skin below the level of the section is followed by 

 co-ordinated movements. These are evidently brought about 

 through direct connections within the spinal cord between the 

 dorsal and ventral roots either of one spinal nerve or of neighbour- 

 ing nerves. This is known as reflex action. Such responses, of 

 which the well-known scratch reaction of the dog is an example, 

 occur in all animals. The integration of reflexes and their purpose- 

 ful control is performed by centres at various levels in a function- 

 ally superposed series, the cerebral cortex being the ultimate one 

 and having become increasingly dominant in the mammalian scale. 



The Spinal Cord 



The spinal cord reflects in its form the basic architectural 

 pattern of the vertebrate central nervous system, being developed 

 in the embryo as a tube and retaining this condition throughout 

 life. The inner part of the wall of the tube is composed of grey 

 matter, the outer part of white matter. The cavity, how^ever, is 

 reduced to a very slender central canal while the walls become 

 enormously thickened by proliferation of the cells and their fibre 

 extensions, through which are established the nervous functions 

 of the system, as a connected conducting mechanism. The cord 

 traverses the vertebral canal, showing slight enlargements in the 

 cervical and lumbar regions in relation to the nerve supply of the 

 limbs, and at about the level of the second sacral vertebra narrows 

 into the slender, thread-like filum terminale, by which it is con- 

 tinued almost to the middle of the length of the tail. 



The sudden tapering of the cord into the filum terminale at the 

 level indicated is a result of growth relatively less than that of the 

 surrounding parts, the cord in the embryo extending through the 

 region occupied in the adult by the filum. Such relative shortening 

 of the cord by retardation of growth is more marked in some 

 animals than in others, the lower tip of the human spinal cord, for 

 example, being usually within the first lumbar vertebra. 



