THE NERVOUS SYSTEM 



303 



561. How Automatic Centers work. As the blood be- 

 comes deficient in oxygen and charged with carbon dioxid, 

 the respiratory center is irritated and sends out im- 

 pulses which cause deeper breathing. This improves the 

 condition of the blood, and the respiration is quieter until 

 the blood again loses oxygen; then stronger impulses are 



Face Sensory -j 



Face Motion 



Taste 



- Spinal Cord 

 1st Spinal Nerve 

 2d Spinal Nerve 



FIG. 201. Diagram of the Twelve Cranial Nerves. 



There are twelve pairs of cranial nerves which arise from the brain and go out through holes 

 in the lower wall of the skull. (How many pairs of spinal nerves are there ? ) The cranial 

 nerves are numbered, according to the location of their roots, from above downward. 



sent, and so on, thus regulating the condition of the blood 

 automatically. The size of the blood vessels is regulated 

 in the same way by the vasomotor center in the medulla, 

 increase of carbon dioxid causing contraction of the sur- 

 face vessels, decrease of it causing relaxation. Automatic 

 acts take place in series. Reflex acts, as for example, 



