THE DOGFISH 25 



anatomy), portions of which have already been seen (p. 12) 

 distributed on the stomach. 



Add all these nerves to the sketch, including II, III, IV, 

 and VI. 



Brain. Cut off the olfactory nerves, lift up the cerebrum and 

 see the intracranial parts of the optic nerves. Cut these close 

 to cranium and continue to lift, noting and cutting the internal 

 carotid arteries (see p. 17) as they pass from the cartilage to the 

 brain. Now vertically below the optic lobes will be seen the 

 roots of the oculomotor nerves. Do these come from the mesen- 

 cephalon or the medulla oblongata? Cut in succession the roots 

 of nerves V, VII-VIII, IX, and X on either side and lift the 

 brain very carefully, noting the origin of the abducens nerve and 

 taking great pains not to injure the prominent structures beneath 

 the thalamencephalon. Finally, cut the spinal cord and remove 

 the brain from the cranium. Insert the parts of the nerves II, 

 III, and VI not already drawn in the sketch of brain and cranial 

 nerves. 



Study and draw the brain from the side and from the ventral 

 surface, making out the regions already seen, as well as the fol- 

 lowing structures: (1) The optic chiasma, or crossing of the opti3 

 nerves beneath the diencephalon ; (2) the lobi inf eriores, a pair 

 of prominences on the ventral surface just behind the chiasma; 

 (3) an unpaired part, the infundibulum, lying between the inferior 

 lobes, expanded behind into a pair of thin-walled sacs, the sacci 

 vasculosi, and bearing on its ventral surface a narrower body, the 

 hypophysis or pituitary body. Note the origin of the oculomotor 

 nerves just in front of the vascular sacs. On the ventral surface 

 of the mesencephalon is a longitudinal groove, continued as the 

 ventral fissure on the spinal cord. This groove separates a pair 

 of broad bands, the crura cerebri, which extend forward either 

 side of the infundibulum just dorsal to the sacci vasculosi, while 

 each cms is extended backwards beneath the medulla as one of 

 the anterior pyramids. 



Add to the sketch of the ventral surf ace. of the brain the fol- 

 lowing blood-vessels. The internal carotids reach the brain 

 on either side just behind the optic chiasma, dividing at once 

 into anterior and posterior branches. The anterior branch sends 

 a main trunk, the middle cerebral artery, which runs along the 

 ventral surface of the cerebrum and the olfactory lobe; and an 



