XXV] BRAIN 647 



proportion to the hind-brain or cerebellum. The former overlap 

 completely and conceal the thalamencephalon and the mid brain, 

 and they are connected with one another by a great transverse band 

 of nerve-fibres, called the corpus callosum. It is customary to 

 map out the surface of the hemispheres into regions, in order to 

 facilitate description in delimiting the areas concerned with the 

 development of specific sensations and with the control of specific 

 movements. These regions are called frontal, parietal, occipital, 

 and temporal lobes. The temporal lobe is separated from the 

 frontal by a deep groove, called the Sylvian fissure (11, Fig. 

 319, A). How well the increased size of the cerebrum is re- 

 flected in the shape of the cranium will be seen when it is 

 recollected that the frontals and parietals, which represent merely 

 the membrane covering the anterior fontanelle, not only form the 

 roof of the cranium but a large part of its domed side wall ; and 

 further that the orbitosphenoid and alisphenoid, which are portions 

 of the cartilaginous brain-case, are restricted to the base of the 

 skull. The cerebrum has in fact protruded through the anterior 

 fontanelle, pushing the membrane before it. The same condition is 

 observable in Birds, but not in Reptiles or Amphibia. The cere- 

 bellum however is also well developed, just as in Birds, having 

 indeed in addition to the lateral lobes an outer pair of lateral pro- 

 jections, called flocculi, embedded in a hollow of the bone that 

 covers the inner ear (Fig. 319). The two halves of the cerebellum 

 are connected with one another by a conspicuous band of fibres in 

 the floor of the brain, called the pons Varolii. 



The nose, except in aquatic Mammalia, is a highly developed 

 sense-organ. The epithelium lining it is produced into scroll-like folds 

 which are supported by thin plates of bone arising from the meseth- 

 moid, and called ethmoturbinals. Above, where the mesethmoid 

 joins the orbitosphenoid, so numerous are the apertures in it to 

 allow the bundles of nerve-fibres from the olfactory cells to pass to 

 the brain, that this part of the bone is reduced to a sieve, whence 

 it has received the name of cribriform plate. From the maxilla, 

 which forms the outer wall of the lower part of the nasal tube, a 

 similar scroll-like bone, the maxilloturbinal, arises, which supports 

 a corresponding fold of epithelium. This fold however is supplied 

 only by the second division of the fifth nerve, and is not believed to 

 have any olfactory function, but merely to act as a filter to free the 

 inrushing air from grosser particles before it reaches the delicate 

 olfactory epithelium. 



