DISSECTION OF THE SHARK 37 



tion and development of the brain and in part by physiological 

 convenience. 



The brain is separated by a constriction (the isthmus) in 

 front of the cerebellum into the cerebrum in front and the 

 rhombic brain (rhombencephalon) below, the latter being 

 further subdivided into the medulla oblongata and cerebellum. 

 The cerebrum is further subdivided into (1) the midbrain 

 (mesencephalon) containing the optic lobes dorsally and the 

 cerebral peduncles and motor centers for eye movements 

 ventrally, (2) the between-brain (diencephalon) containing 

 thalamus, epithalamus, and hypothalamus, and (3) the end- 



fourth ventricle >^_T2T^'\^ -somatic sensory 



column 

 -visceral sensory 



branchial ganglion. \^^&-^J :; -::-J column 



'-visceral motor 

 column 



pharyngeal nerve^^f/^ \_ J^_^><^ somatic motor 



..... column 



pre-branchial 



FIG. 6. Diagrammatic cross-section through the medulla oblongata of 

 the dogfish in the region of the vagus nerve to illustrate the innervation 

 of the gills and the arrangement of the functional columns of the oblongata. 

 The groove between the visceral motor column and the visceral sensory 

 column is the sulcus limitans. 



brain (telencephalon) comprising the anterior end of the brain 

 tube, the cerebral hemispheres, and olfactory bulbs. Locate 

 these regions and compare the fuller study of the subdivisions 

 and development of the brain in Section 40. 



27. Now remove the brain from the cranial cavity, first care- 

 fully cutting the nerve-roots so as to permit their subsequent 

 identification and" avoiding injury to the olfactory bulbs in 

 front and the pituitary body on the ventral surface. Examine 

 the ventral surface and complete the study of the parts listed 

 in Section 26. 



28. Make a transverse section across the medulla oblongata 

 at the level of the roots of the VIII nerves. Compare this sec- 

 tion with a similar one made in the vagus region (Fig. 6) and 

 note the differences. 



