480 THE DOGFISH CHAP. 



side you have not already dissected so as to expose the membranous 

 labyrinth. Examine under water, and make out the vestibule with its 

 contained otolithic mass, the three semicircular canals with their 

 ampulla, and the branches of the auditory nerve (compare Fig. 59). 



III. Now examine the preserved brain from above, from below, and 

 from the side, making out, in addition to the parts already noticed 



(F, I, i)- 



1. The optic chiasma, infundibuhim (with an oval lobe and a vascular 

 sac on either side), pituitary body, crura cerebri, and, as far as possible, 

 the origins of the nerves. Sketch from below. 



2. On one side of the brain, cut into the olfactory lobe, prosen- 

 cephalon, optic lobe, and cerebellumjfrom above, so as to expose the olfac- 

 tory ventricle, lateral ventricle, optic ventricle, and cerebellar ventricle. 

 Then bisect the entire brain into right and left halves with a sharp 

 scalpel, and examine the uninjured half in longitudinal section, noting, 

 in addition to the parts mentioned above, the third ventricle, foramen of 

 Monro, iter, and fourth ventricle. Sketch. 



G. Transverse Sections. Cut thick transverse sections of an en- 

 tire dogfish with a knife through a. the anterior, and b. the posterior 

 part of the head (pharyngeal region) ; c. about the middle of the 

 body ; and d. the tail. Make out the relations of the various parts and 

 organs, and sketch the lateral half of each section. 



A more satisfactory method than this is to obtain a very young dog- 

 fish, not more than \ inch in diameter, and after cutting it transversely 

 into pieces about | inch in thickness in the regions named above, stain, 

 imbed, and mount a few sections from each piece (see p. 136). These 

 can first be examined with a lens or with the low power of the micro- 

 scope, and then, by putting on the high power, important points in the 

 histology can be made out. In addition to the minute structure of the 

 tissues and organs described in Part I. of this book, the structure of the 

 notochord (p. 441), integumentary sense-organs (pp. 432 and 464), der- 

 mal teeth (p. 433 and 445), &c. , should be studied. 



H. Side Dissection. It is very instructive to supplement and 

 recapitulate your work on the anatomy of the dogfish by dissecting 

 another specimen from the side (compare Fig. 117), as in the case of the 

 crayfish. Cut open the abdominal cavity as before, very slightly to the 

 left side of the middle line. Continue the cut forwards through the 



