giving rise to nerve fibers. Also located in the floor of the 

 mesencephalon is the functionally important red nucleus. 

 Here the brain stem also contains fiber tracts connecting 

 liigher and lower parts of the brain. These tracts are de- 

 scribed as the peduncles. 



The metencephalon has a dorsal cerebellum and a ven- 

 tral pons as well as the usual ascending and descending 

 fiber tracts of the brain stem. The myelencephalon has a 



membranous roof, a chorioid ple.xus, and a thick basal por- 

 tion consisting of bilateral reticular bodies, olivary nuclei, 

 and fiber tracts. 



Cranial nerves of the mammal A description of the brain 

 is quite incomplete without the cranial nerves which stem 

 from it. These are ten to fifteen in number depending on 

 how and in what animal one approaches them. Mammals 



habenular commissure 

 mossa intermedia 

 foramen of Monroe 



pallia! commissure 



fasciculus dentotus, 

 anterior commissure 



olfactory bulb 



pineal 



posterior commissure 



^corpora quodrigemino 



PLATYPUS 



cerebellum 



olfactory tuber 



piriformis 

 optic nerve (II) p.,^.,^^^ 



medulla oblongata 



fourth ventricle 

 abducens nerve (VI) 

 motor division of V 

 trigeminal nerve (V) 



anterior colliculus 



posterior colliculus 



saccus dorsalis 

 habenula 

 cerebral hemisphere^ 

 pallial commissure 

 olfactory bulb 



anterior I ■ i r i_ ii 



I Hobes ot cerebellu 



posterior J 



medulla oblongata 

 queduct 



vomeronasal nerve 



anterior commissure 



optic chiasma' 



B OPOSSUM massa intermedia 



pituitary 

 third ventricle 



anterior colliculus (optic lobe) 

 habenula 

 intermediate mass 

 corpus callosum 



fornix. 



anterior commissure 

 olfactory bulb 



RABBIT 



optic nerve 



third ventricle / pituitary 

 hypothalamus 



posterior colliculus 



cerebral aqueduct 

 cerebellum 



fourth ventricle 



medulla oblongata 



Figure 1 3-2. Semidiagrammatic medial views of right brain halves of three mammals. (A after Neal 

 and Rand, 1936; B after Romer, 1955; and C after Young, 1957) 



THE CONDUCTING AND INTEGRATING SYSTEM 



385 



