io2 VERTEBRATES WITHOUT JAWS iv. 9- 



ventricles. Presumably the vascular membranes of the brain are highly 

 developed in lampreys because of the absence of cerebral blood 

 vessels. 



From the lower part of the mid- and hind-brain arise all the cranial 

 nerves except the olfactory and optic. These nerves follow the same 

 plan as those of gnathostomes but they are difficult to make out by 

 dissection in the lamprey and will be left for consideration in con- 

 nexion with the dogfish, in which they can easily be dissected. The 



beet. opt. 

 chor. pL3 



chor.pt, 4. 



otf.ep — IM-<^9 % 



/nterped. 



Fie. 65. Sagittal section through head of lamprey. 



cereb. cerebellum; cer.h. cerebral hemisphere; chor.pl. 3 & 4, choroid plexuses of the 3rd and 

 4th ventricle, extending also into the midbrain; h.s. naso-hypophysial tube; hab. habenular 

 region; hyp. hypothalamus; interped. interpeduncular region; med. medulla oblongata; Mull. 

 M Oiler's cell; not. notochord; o. glandular organ of nasal sac; olf.ep. olfactory epithelium; 

 olf.n. olfactory nerve; p. ant., p. int., and p.nerv. partes anterior, intermedia, and nervosa of the 

 pituitary gland; parap. parapineal; pin. pineal; ted. opt. tectum opticum. 



cranial nerves represent nerves similar to the dorsal and ventral nerve- 

 roots of the trunk, much modified as a result of the special develop- 

 ment of the head (p. 148). They carry afferent fibres from the skin of 

 the head and gills and motor-fibres for moving the eyes, sucker, and 

 branchial apparatus. 



From the relative sizes of the parts of the brain it can be seen that 

 the various special sensory centres are still small. The largest part of 

 the brain is the medulla oblongata, which is well developed because 



