222 



ELEMENTS OF BIOLOGY 



of nerve fibres that make up even the most simple of vertebrate 

 brains. A response to a stimulus that follows such an abbreviated 

 path is known as a reflex act and the path over which it travels is 

 termed a reflex arc (Fig. 150). 



Cranial Nerves. A number of nerves enter directly into the 

 vertebrate brain without any association with the spinal cord. 

 These are termed the cranial nerves. There are ten pairs in fishes 



ASSOCIATION 

 NEURONS 



SENSORY 

 FIBER 



NEURON 



RECEPTOR 



SENSORY 

 GANGLION 



MOTOR FIBER 



MUSCLE 



Fig. 150. — Scheme of a reflex arc. (After Kiihn: Gntudriss der aUgeweinen Zoologie. 



Georg Theime, Leipzig.) 



and Amphibia and twelve pairs in reptiles, birds and mammals. 

 The fibres that compose them are outgrowths from the nerve 

 cells of the brain and nearby ganglia, but in the case of the 

 optic and of the olfactory nerves the developmental origin is from 

 specialized regions of the fore-brain that during development grow 

 out to become parts of the nares and the eye. The cranial nerves are 

 not segmental structures. Some are purely sensory, some are purely 

 motor, and some are made up of both sensory and motor fibres. A 

 tabulation is given showing the name of each cranial nerve, its 

 region of connection with the brain, its distribution and function in 

 Man. 



