DISSECTION OP THE SHAKE 35 



with corresponding modifications of the IX and X pairs of 

 nerves; the presence in the dogfish of an extensive special 

 system of subcutaneous sense organs structurally (and prob- 

 ably physiologically) related to those of the internal ear. 

 These are the sense organs of the lateral line canals and 

 ampullae of Lorenzini, for which there is a special system of 

 nerves, the lateralis components of the VII, IX, and X nerves 

 (see Section 18). 



Draw the dissection of the brain and cranial nerves, indi- 

 cating the functional components in each peripheral nerve- 

 trunk by colors in accordance with the scheme given in Section 

 62 (p. 62), coloring the lateral line nerves the same as the VIII 

 nerve (brown or green). 



22. Complete the exposure of the brain, carefully preserving 

 the roots of the cranial nerves. 



Now, viewing the brain from above, review the arrangement 

 and physiological composition of the cranial nerves and note 

 particularly the part of the brain with which each peripheral 

 end-organ or group of physiologically similar organs is related. 

 Simple inspection shows that the organs of smell are connected 

 with the olfactory bulbs, and in fact these and almost the 

 whole of the cerebral hemispheres and epithalamus and hypo- 

 thalamus form the olfactory part of the brain (stippled in Figs. 

 2 and 3) . In the same way the eyes are related with the optic 

 lobes of the midbrain (obliquely cross-hatched in Figs. 2 and 3), 

 the ears and lateral line organs with the acoustico-lateral area 

 (cross-hatched with horizontal lines), and the nerves of general 

 skin sensibility with the general cutaneous area (unshaded). 

 Locate precisely these areas on your specimen. 



23. Next carefully remove the membranous roof of the 

 fourth ventricle and study the floor of the fourth ventricle, 

 noting the following structures, passing from the median sulcus 

 laterally (cf . Figs. 2 and 6) : (1) a longitudinal ridge marking 

 the position of the fasciculus longitudinalis medialis (" pos- 

 terior longitudinal fasciculus") and farther ventrally the posi- 

 tion of the ventral gray column of the spinal cord and nuclei 

 of the VI, IV, and III cranial nerves this is the somatic motor 

 column; (2) a wide longitudinal groove parallel with the last, 

 below which are found the motor nuclei of the X, IX, VII, 



