THE RABBIT 309 



The hypoglossal nerve, the twelfth spinal nerve, arises by several 

 roots near the midventral line of the medulla and is distributed 

 to the muscles of the neck and the tongue ; it is the motor nerve 

 of the tongue. 



Exercise 31. Draw a semidiagrammatic view of the lateral aspect of 

 the brain, showing the cranial nerves, so far as observed. 



Study the ventral surface of the brain. Separate the brain from 

 its attachments and remove it from the body. In taking it out 

 note the hypophysis, a median projection back of the optic nerves ; 

 it is lodged in a depression of the floor of the cranial cavity and 

 may be torn off in removing the brain. 



At the anterior end of the hemispheres are the olfactory lobes ; 

 and back of them on each side is located the olfactory tract, which 

 passes back to the pyrif orm lobe, the posteroventral portion of the 

 hemisphere. 



These structures belong to the telencephalon, or cerebrum, the 

 anterior division of the brain. Back of them are those which be- 

 long to the diencephalon, the second division. The optic nerves 

 spring from the optic chiasma, which is formed by the meeting of 

 the optic tracts, one of which comes from each side. Back of the 

 chiasma is the tuber cinereum, a median swelling from which pro- 

 jects the infundibulum, bearing at its distal end the hypophysis, or 

 pituitary body. Back of the tuber cinereum is a paired elevation, 

 the mammillary body. 



The ventral surface of the mesencephalon, the third division of 

 the brain, is formed by the cerebral peduncles, a pair of rounded 

 elevations which lie immediately back of the mammillary body, 

 converging toward the median line. The oculomotor nerves spring 

 from this region. 



The pons, the ventral surface of the metencephalon, which forms 

 the fourth division of the brain, is divided by a median groove into 

 two lateral portions, and connects the two sides of the cerebellum; 

 at the lateral border of its lateral ends the small trochlear nerves 

 arise. 



The ventral surface of the myelencephalon, the fifth division of 

 the brain, is formed by the two trapezoid bodies, one on each side 



