i go VERTEBRATE ZOOLOGY 



brain is removed. Back of the infundibulum are the two oculo- 

 motor nerves; they spring from the midbrain near the median 

 line. The medulla oblongata extends from the midbrain to the 

 spinal cord; near the median line at the anterior end of the 

 medulla will be seen the abducens nerves. 



Exercise 31. Draw a view of the ventral aspect of the brain. 



Study the cavities of the brain. The brain is a hollow structure 

 and contains a series of cavities which are a continuation of the 

 central canal of the spinal cord. The cavities of the two hemi- 

 spheres are called the lateral ventricles, or the first and second 

 ventricles. Press the hemispheres slightly apart and cut through 

 the thin inner and posteromedial wall of one of them into the 

 ventricle. Remove a portion of the wall and observe the cavity. 

 Note, on its ventral surface, the large white mass which almost 

 fills it ; this is the corpus striatum. In the medial side of the pos- 

 terior wall is the foramen of Monro, by which the lateral ventricles 

 communicate with each other and with the third ventricle, which 

 is in the diencephalon. 



Remove the cerebellum by carefully cutting the right and left 

 peduncles, which join it with the medulla on each side. Connect- 

 ing the two optic lobes will be seen a bridge of white fibers called 

 the optic commissure ; in front of this is the roof of the third ven- 

 tricle ; behind it is the roof of the fourth ventricle. Remove the 

 former and note the narrow space, the third ventricle, beneath, 

 the thickened lateral walls of which are the optic thalami. Probe 

 the ventricle and note its depth. Cut an opening in one of the 

 optic lobes and note its ventricle. Connecting the third and the 

 fourth ventricle is a median canal called the aqueductus Sylvii; 

 the ventricle of the optic lobe joins it on each side. The fourth 

 ventricle is broad and shallow. 



Exercise 32. Draw a diagram showing the shape and position of the 

 ventricles. 



The Muscular System. Kill a fresh animal. Make a midven- 

 tral slit through the skin from the mouth to the anus, and skin 

 the ventral surface of the body, including the wings and the legs. 



