276 STRUCTURE OF THE VERTEBRATES 



dendrites are short and enter the cell body at several points, 

 or poles, and pass out through the single axone. In the unipolar 

 cells the dendrites are very long and the axone short. The 

 relative length of the processes in the two types becomes clear 

 if position and function are considered. The cell bodies lie in 

 or near the spinal cord, and are connected with all the regions 

 of the body through their neurofibrils. The sensory cells have 

 long dendrites in contact with the periphery of the body, and 

 short axones to the cord. The motor cells, lying in the cord, have 

 short dendrites and long axones. The cells of the brain may 

 vary in the relative length of dendrites and axone. 



The axones give off collateral branches during their course. 

 Axones and their collaterals wliich pass from and to the cord 

 are usually covered with a fatty myelin sheath. The lipoid sub- 

 stance is enclosed in a delicate connective tissue sheath which is 

 broken at irregular nodes. The sheath between the nodes is 

 continuous and formed of two concentric tubes with the space 

 between filled with the myelin substance. The cell bodies are 

 bare. The unmyelinated fibers are those of the association 

 neurones which do not leave the brain or cord, and the fibers 

 of the sympathetic system. 



Unmyelinated tissues have a pale, translucent appearance 

 when cut and are known as the gray substance of the nervous 

 system. The myelinated fibers have a waxy white appearance 

 and form the white substance. A trans-section of the brain or 

 cord will demonstrate the two types of neural substance. 



Cell bodies are usually collected into definite masses. Any 

 group of cells which is separated from other groups is a gang- 

 lion; and the groups of nerve cells which develop from the 

 neural crests are therefore the spinal ganglia. If, however, cell 

 groups are continuous or barely separated from one another, as 

 in the brain and cord, they are termed nerve nuclei. 



The functional unit of the nervous system is made by three 

 or more neurones forming a reflex arc. External stimuli are 

 usually received by specialized sensory cells, the receptors of 

 sensations. The impulse is carried from the receptor to the den- 

 drite of a sensory neurone, and is transmitted through the spinal 

 ganglion into the cord. Small association neurones take the 

 stimulus from the dorsal region of the cord to the motor cells 



