THE SPINAL BULB 



231 



example, when a muscle is active, the sympathetic sys- 

 tem dilates the blood-vessels in the muscle so as to allow 



more blood to go to it 

 than when it is at rest; 



factory nerve when the blood pressure 



falls, it makes the heart 

 beat faster; when the 

 pressure rises, it makes 



'Porebrain (hemispheres) 

 Midbrain (optic lobes) 

 Hindbrain (cerebellum) 

 Afterbrain (Medulla oblongata) 



it beat more slowly; 

 when the skin is hot, it 

 dilates the blood-vessels 



FIG. 125. The brain of a fish. 

 Steiner.) 



(After 



and starts the secretion of per- 

 spiration; when food enters the 

 stomach, it stimulates its glands 

 and muscles to activity; and so 

 on throughout the unconscious 

 adjustments made in the organs 

 of the body. 



Spinal bulb. In its upper 

 part, the spinal cord is enlarged 

 into the spinal *bulb, or medulla 

 oblongata. The spinal bulb 

 contains several important 

 nerve centres which aid in con- 

 trolling certain of the vital or- 

 gans, as the heart. Since the 

 spinal bulb lies entirely within 

 the skull and its structure is 

 much more complex than that 

 of the cord, it is considered to 

 be a part of the brain. As a 



Olfactory j. 

 nerve 



Forebrain 

 ( hemispheres) J 



Pineal body. 

 Midbrain 

 (optic lobes) - 



Hindbrain 

 (cerebellum )- 



VII and VIII- 

 Afterbram 



(spinal bulb)- 



ix- 



X> 



XII- 



xr 



Spinal cord- 



FIG. 126. The brain of a lizard, 

 with the cranial nerves in- 

 dicated by numerals. (After 

 Wiedersheim.) 



