THE DOGFISH 19 



Exercise 18. Draw a side view of the brain, showing the cranial 

 nerves so far as they have been observed. 



The Ventral Surface of the Brain. Remove the two sides of 

 the skull. Separate both olfactory lobes from their capsules and 

 bend the cerebrum back, exposing its ventral surface. Press the 

 side of the brain away from the under portion of the skull and 

 note the small trochlear nerve, which issues from the ventral 

 surface of the optic lobe and enters the orbit, and also the small 

 abducens nerve, which issues from, the medulla and also goes to 

 the orbit. Cut these nerves. Cut the optic nerves and note back 

 of them a pair of lobed bodies, the lobi inferiores, between which 

 is a median projection, the infundibulum. Extending backward 

 from the infundibulum is the hypophysis, or pituitary body, which 

 fills a deep depression in the ventral wall of the skull. All these 

 structures belong to the diencephalon. Back of the infundibulum 

 the ventral surface of the brain is without important special 

 structures. 



Exercise 19. After removing the brain from the body, draw the 

 ventral aspect on a scale of 2 or 3 and carefully label all the 

 organs observed. 



The Ventricles of the Brain. The brain is a hollow structure 

 and contains a series of spaces which are continuous with the 

 central canal of the spinal cord. In the two hemispheres are two 

 large spaces called the first and second, or the lateral, ventricles. 

 Make a transverse incision across the hemispheres and observe 

 them. In the diencephalon is the third ventricle, which joins the 

 first two at its forward end. The roof of the third ventricle is the 

 thin dorsal wall of the diencephalon, which has already been men- 

 tioned. The fourth ventricle is in the medulla, where it occupies 

 the fossa rhomboidalis, the roof of which is likewise very thin. 

 The third and fourth ventricles are joined by a canal called the 

 aqueductus Sylvii ; this canal passes between the optic lobes and 

 communicates with the large space in each of them. Bisect the 

 brain and observe these spaces. 



Exercise 20. Draw a diagram representing the cavities of the brain. 



