THE RABBIT 455 



corpus albicans. The mid-brain is almost covered by the 

 cerebral hemispheres. Each of its optic lobes is divided 

 into two by a transverse furrow, so that four corpora 

 quadrigemina or colliculi arise. The crura cerebri are more 

 prominent than in the frog. In the hind-brain, the 

 cerebellum is very large and much-folded and consists of a 

 median lobe or vermis and two lateral lobes, each of which 

 bears on its outer side a small lobe known as the flocculus. 

 The lower side of the hind-brain is crossed by a wide, fiat 

 band of transverse fibres, the pons Varolii, which connects 

 the two halves of the cerebellum. The medulla oblongata 

 is broad in front and narrows gradually backwards to 

 become the spinal cord. It is marked by a ventral fissure 

 bordered by two longitudinal bands or pyramids. 



The cranial nerves are twelve in number. The first ten 

 resemble those of the frog in origin and function, but show 

 certain differences ; thus the olfactory nerves arise as a 

 number of fine threads directly from the olfactory bulb and 

 pass at once through the openings of the cribriform plate at 

 the front end of the cranium. The seventh nerve has no 

 ophthalmic branch. The eleventh or accessory nerve arises 

 from the side of the medulla and spinal cord by a number 

 of roots, the first of which is just behind the vagus and 

 the last at the level of the fifth spinal nerve. It supplies 

 certain muscles of the neck. The twelfth or hypoglossal 

 nerve also arises by several roots ; these are situated on the 

 ventral side of the medulla, outside the pyramid. Its course 

 resembles that of the hypoglossal (first spinal) nerve of the 

 frog. The spinal cord and nerves do not differ essentially 

 from those of the frog. The sympathetic system is upon 

 the same general plan as that of the frog. It has two 

 ganglia on each side in the neck, twelve pairs in the thorax, 

 and the same number in the abdomen. From the hinder 

 thoracic ganglion there starts a splanchnic nerve, which 

 passes backwards into the abdomen and ends with its 

 fellow in a group of coeliac ganglia around the anterior 

 mesenteric artery. These ganglia, with numerous nerves 

 uniting and branching from them, constitute the solar 

 plexus (Fig. 418). A smaller plexus and ganglion of the 

 same kind lie around the origin of the posterior mesenteric 

 artery. A number of important nerves belonging to all 



