xvn THE NERVOUS SYSTEM 187 



of reflex action carried out not by the spinal cord alone, but 

 by the intervention also of the brain, and in the first of the 

 instances given, of the brain exclusively. Many movements 

 of ordinary life which are started by the will, and therefore 

 voluntary, are often continued reflexly. Thus we can go on 

 walking without thinking about it, every step l>cing properly 

 performed, the necessary muscles contracting because they 

 receive impulses from the central nervous system, regulated 

 in accordance with sensory impulses received by the central 

 nervous system, from the eye, or the car, or due to the con- 

 tact of the legs themselves with the ground. Again, mainly 

 involuntary movements, such as those of respiration, are carried 

 out or modified by reflex action either of the spinal cord or 

 of some part of the brain. 



The Brain 



The brain is a large organ consisting of several parts. The 

 lowest part, called the spinal bulb or medulla oblongata, 

 is continuous with the spinal cord, and is somewhat like it in 

 structure, but is' larger iu diameter, gradually increasing in 

 width from the spinal cord upwards. The bulb in its natural 

 position gradually bends forwards, so that the brain lies rather 

 in a fore-and-aft direction than in an up-and-down direction 

 like the spinal cord. Springing from the sides of the upper 

 part of the bulb is a large mass which lies on the dorsal or 

 hinder side of the bulb, and largely overlaps it downwards. 

 Thi-. is called the cerebellum; its surface is closely folded 

 or plaitt-d. The cerebellum is connected to the bulb not only 

 by nervous tissue passing into it from the bulb on each side, 

 but also by a large bridge of tissue, consisting mostly of bundles 

 of nerve fibres passing from one side to the other across the 

 front of the upper part of the bulb. This bridge of tissue is 

 called the pons. Some of the bundles of nerve fibres which 

 make up the white matter of the spinal bulb pass from the 

 bulb into the cerebellum, while other bundles go straight on 

 past the poiis, forwards and upwards, connecting the bulb with 

 the parts of the brain in front. Just above the pons these 

 bundles of fibres appear .is two columns, called the crura 

 cerebri, one on each .side at the base or lower side of the 



