CHAPTER VII. 



1. Nerve-fibres and nerve-cells; 2. Irritability of nerve-fibre; 

 3. Transmission of the irritation; 4. Isolated ' transmission ; 

 5. Irritability ; 6. The curve of irritability ; 7. Exhaustion and 

 recovery, death. 



1. In the body of an animal nerves occur in two forms : 

 either. as separate delicate cords which divide into many 

 parts and distribute themselves throughout the body, 

 or collected in more considerable masses. The latter, 

 at least in the higher animals, are enclosed in the bony 

 cases of the skull and vertebral column, and are called 

 nerve-centres, or central organs of the nervous system ; 

 the nerve-cords pass from these centres to the most 

 distant parts, and are spoken of as the 'peripheric nerve- 

 system. When examined under the microscope these 

 peripheric nerves are seen to be bundles of extremely 

 delicate fibres united into thicker bands within a mem- 

 brane of connective tissue. Each of these nerve-fibres 

 when examined in a fresh state, and enlarged 250 or 

 300 times, is exhibited as a pale yellow transparent 

 fibre in which no further differentiation is visible. The 

 appearance of the fibre soon, however, changes ; it be- 

 comes less transparent, and a part lying along the axis 

 becomes marked off from the circumference. This inner 

 part is usually flat and band-like, and when seen under 

 a higher power exhibits a very minute longitudinal 



