DISSECTION OF THE SHARK 35 



lum is a specialized derivative of it. The longitudinal groove 

 between columns 2 and 3 is the sulcus limitans (cf. the develop- 

 ment of the human brain, Section 38). It separates motor 

 centers below (ventrally) from sensory centers lying farther 

 dorsally. 



24. From these observations it appears that the brain shows 

 various external thickenings or enlargements, each of which is 

 related to a particular physiological type of end-organ. We 

 may, in fact, recognize a "nose brain," "eye brain," "ear brain," 

 etc., and in addition the cerebellum above these primary sensori- 

 motor centers. There is no cerebral cortex. 



25. Draw the medulla oblongata from the dorsal side, twice 

 natural size, after removal of the membranous roof, to illustrate 

 the functional areas in the walls of the fourth ventricle. In 

 this drawing the functional columns should be tinted to cor- 

 respond with the colors used in the drawing of the peripheral 

 nerves (Section 21). Thus, the acoustico-lateral area will be 

 colored the same as the VIII and lateral line nerves, the gen- 

 eral somatic sensory area the same as the general cutaneous 

 nerves, etc. The drawing of the entire brain called for in 

 Section 21 may also be tinted in the same way. 



26. The physiologically distinct areas noted in the preceding 

 sections tend to be grouped in larger regions, the pattern of this 

 grouping being determined in part by the primitive segmenta- 

 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 (mesen- 

 cephalon) containing the optic lobes dorsally and the cerebral 

 peduncles and motor centers for eye movements ventrally, (2) 

 the between-brain (diencephalon) containing thalamus, epi- 

 thalamus, and hypothalamus, and (3) the end-brain (telenceph- 

 alon) comprising the anterior end of the brain tube, the cere- 

 bral hemispheres, and olfactory bulbs. Locate these regions 

 and compare the fuller study of the subdivisions and develop- 

 ment of the brain in Section 40. 



