388 THE NERVOUS SYSTEM 



The forebrain, — This consists of two elongated bodies 

 lying side by side and called the cerebral hemispheres. 

 Together, they constitute the cerebrum. At their ex- 

 treme front are two swellings, from which a pair of cranial 

 nerves extend to the nose. The fibers of these nerves are 

 afferent, and carry sensor}^ impulses from the nose to these 

 lobes of the brain. They give to the animal the sense of 

 smell. On this account the lobes of the brain from which 

 these nerves arise are called the olfactory lobes, and the 

 nerves are called the olfactory nerves. The olfactory are 

 the only nerves connected directly with the cerebral 

 hemispheres. 



The midbrain. — Just back of the hemispheres is the 

 midbrain. This is covered on the top and sides by two 

 swellings called the optic lobes, so named because the 

 sensory nerves from the eye enter them. They are the 

 receiving points of sensations of light. The optic, or sight 

 nerves (the second pair of cranial nerves) enter these 

 lobes on the under side, and are crossed so that the nerve 

 from the right eye enters the left lobe, and vice versa. 

 These optic lobes also communicate with the forebrain 

 by means of which we wiU speak later, but the optic nerves 

 have no direct connection with any part of the brain except 

 these lobes. 



The third pair of cranial nerves also arises from the 

 midbrain, and, unlike the first two pairs, it carries impulses 

 from the brain instead of to it (that is, it is a motor or 

 efferent nerve). The nerve fibers of this third pair control 

 four of the six muscles which move the eyeballs and also 

 some of the internal eye muscles. They are called the 

 oculo-motor nerves. 



The hindbrain. — The rest of the brain, which extends 



