THE NBET.OrS SYSTEM AND OEGAITS OP SENSE. 219 



cerebral hemisphere a process proceeds forwards. These are 

 the olfactory lobes. Between the cerebral hemispheres and the 

 cerebellum, on the dorsal aspect of the brain, is a small process 

 called the pineal gland. A median inferior prominence is called 

 the pituitary body. Within, the brain is hollow, but the hollow 

 space is variously disposed in different regions, and its walls are 

 of very different thicknesses in different parts. Those parts of 

 the cavity which extend into the hemispheres are called the 

 lateral ventricles, and they open posteriorly by an aperture 

 termed the foramen of Munro into a median cavity called the 

 third ventricle. This is continued backwards into another beneath 

 the cerebellum, which is the fourth ventricle. 



The spinal cord shows two enlargements where it gives off 

 nerves for the wings and legs respectively. The posterior of 

 these enlargements also contains a cavity of ventricle termed the 

 sinus rhomhoidalis. 



Some of the nerves given off from the brain go to the organs 

 of sense, while others go to muscles and viscera. The points 

 of exit from the skuU of some of the nerves have been already 

 indicated *. 



The Nose. — This consists of two nostrils, which open externally 

 in the way before described t and which pass back to open 

 internally on the roof of the mouth, generally by a single aper- 

 ture there. The olfactory lobe (often called olfactory nerve) 

 above mentioned supplies the nerves which give the power of 

 smell, and they ramify on the membrane which lines the nasal 

 cavities. 



The Organ of Taste. — The tongue, which has been already 

 described J, is a part which in other animals serves for gusta^ 

 tion, but probably has little or no power of taste in very many 

 Birds. That sense is probably miuistered to by the lining of 

 the bealj, which is doubtless also a most important instrument 

 for ministering to the sense of touch. 



The Ear. — There is hardly any external ear, save sometimes a 

 circle of. feathers — as in the Bustard, Ostrich, and Owl. In the 

 last-named bird the external opening is wide, and is protected 

 by a flap of skin with a few folds, thus distantly resembling the 

 human ear. The external opening leads into a very shallow 

 cavity bounded by the quadrate, squamosal, and exoccipital bones. 



* See ante, p. 183. t See anU, p. 146. 



J See ante, p. 207. 



