THE RABBIT. 89 



6. The Nervous System. If we watch a living 

 rabbit, we see signs of constant muscular contraction. 

 Eyes, lips, and snout are constantly moving, even when it 

 seems at rest. It grasps and nibbles food, hops slowly or 

 runs from time to time, and even when asleep the heart 

 continues beating, and the thorax-muscles never cease 

 pumping air in and out of the lungs. Every one of these 

 motions in itself involves a complex series of contractions of 

 different muscles. Obviously if the rabbit is to live for 

 even five consecutive minutes, there must be some agent for 

 co-ordinating all these activities, for making them work in 

 harmony, and in such a way as is most serviceable to the 

 whole animal. This agent is the nervous system. The 

 nervous system may be divided into two parts the central 

 nervous system, which is the governing organization of the 

 multicellular community (chap, v., 12), and the peripheral 

 nervous system, which forms the means of communication 

 between the central system and the various organs of the 

 body. In the rabbit the brain and spinal cord constitute 

 the central nervous system, while the nerves and nerve- 

 endings (in other organs) together with certain structures 

 called ganglia, form the peripheral system. 



7. Nerve-cells. The whole nervous system consists of 

 nerve-cells and their branches the largest and longest of the 

 latter forming nerve-fibres connected together by connec- 

 tive tissue.* Nerve-cells are usually classified according to 

 their shape as unipolar, bipolar, and multipolar. We may 

 take the last of these as the most typical form. Such a cell 

 is large (fig. 51, A) and has a large and conspicuous nucleus. 

 Its shape is irregular, - as it runs out into a great many 

 processes. The majority of these branch repeatedly, and 

 form a fine ramification (" arborization "). Such branches 

 are called dendrons. But one branch differing but little in 

 appearance from them is continued on to a relatively 

 enormous length possibly over a foot in the rabbit, and 

 much more in larger animals as the axis-cylinder of a 



* The connective tissue of the central nervous system is called 

 ncuroglid) and differs in certain important respects from all other 

 connective tissues ; bnt these are beyond our present purpose. 



