Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 299 



numbers of myelin-wrappcd fibers that extend up and down the cord. The 

 regions of gray and white matter are continued into the brain, but in the cere- 

 bellum and cerebrum their positions are reversed and thus the cortex or outer 

 layer of the brain, is gray matter. 



The gray matter of the cord contains neurons with various functions. 

 Among them are connecting neurons (adjustors) that transmit impulses from 

 one neuron to another in the same or in different levels; motor neurons, always 

 in the ventral horns of the gray matter, that carry impulses to skeletal muscles 

 and glands; and neurons of the autonomic system that carry impulses to the 

 interna' organs and other structures. 



In the white matter, the axons are segregated in bundles of fibers of similar 

 function. Great numbers of axons of cell bodies in the spinal ganglia carry 

 sensory impulses to the brain. These are the ascending tracts. There are also 

 axons from cells in the gray matter of the brain, carrying impulses to motor 

 cells in the ventral horns of the gray matter of the cord which then relay them 

 to the muscles. These are the descending tracts (Fig. 16.11 ). If we are sud- 

 denly pricked by a pin, we not only jerk involuntarily, which is the reflex 

 action, we also know about the prick and may remove the pin. The apprecia- 

 tion of the prick and removal of the pin depend on the sensory impulses to the 

 brain, the association of cells in the higher centers, and a complex of impulses 

 to the muscles. In part these relationships have been found out by observing 

 symptoms in persons whh injured nerve cords and correlating these with de- 

 stroyed tracts found when the spinal cord was examined after the patient's 

 death. With some diseases the patient cannot locate his arms and legs without 

 looking at them and must watch his feet in order to walk. This is due to the 

 destruction of the nerve cells responsible for transmitting the sense of position 

 of muscles and joints to the central system. 



Vertebrate Brain 



The brain is the master coordinator of the bodily activities of an animal and 

 of its awareness and adjustment to the environment. The brain and chief sense 

 organs are appropriately so located that wherever the animal travels, they 

 lead on and arrive there first. Every creeping baby has that experience. 



General Description. The vertebrate brain is the bulbous front end of a 

 tube whose walls are composed mainly of nerve cells (Fig. 16.17). In fishes 

 and other lower vertebrates, its outer surface is smooth and the cavity within 

 it is relatively large in comparison to the thickness of the walls. In mam- 

 mals, and especially in man, its surface dips and bulges and the cavity within 

 it is relatively small compared to the thickness of the walls. The great pile 

 of nervous tissue that makes up the cerebral hemispheres is a comparatively 

 late development in animal history, and the cerebral cortex with its billions 

 of interrelated neurons is largely a mammalian achievement (Fig. 16.18). 



