Chap. XL] THE BRAIN AND NERVES. 221 



(anterior genu), doing the same posteriorly (posterior genu\ w4|eS/: v 

 it comes into relation with the fornix. Between the two is .. 

 an area called the septum lucidum. It is formed of the inner 

 walls of the lateral ventricles of this region. In higher types 

 this area is more completely enclosed by the bending down of 

 the more marked anterior genu of the corpus callosum. In this 

 way a space between the two walls is more or less shut in, and 

 constitutes the so-called fifth ventricle, which is, however, it 

 should be noted, outside the brain. Anteriorly the cerebral 

 hemispheres give rise, as outgrowths, to olfactory lobes. In the 

 frog the so-called olfactory lobes (olf.) are relatively large, their 

 bases being closely united in the middle line. They contain 

 minute cavities, w^hich are prolongations of the lateral ventricles. 

 Anteriorly they become nerve-like cords. It is very probable, 

 however, that these so-called olfactory lobes are the anterior 

 parts of the cerebral hemispheres. In the pigeon the olfactory 

 lobes (olf.) are small and apposed, but not united in the middle 

 line. They send forward nerve-like cords. In the rabbit the 

 olfactory lobes (olf.) are small (though larger than is usual in the 

 mammalia), and are not united in the mid-line. They send out 

 many fibres through the cribriform plate. 



Cranial Nerves. In the frog there are ten pairs of cranial 

 nerves : 



I. Olfactory ; sensory nerves derived from the olfactory lobes 

 and passing to the organ of smell. 



II. Optic ; sensory nerves arising as outgrowths of the thalam- 

 encephalon, but acquiring a secondary connection with the 

 optic lobes, and passing to the organ of vision. The fibres of 

 the optic nerves interlace at the chiasma. 



III. Oculo-motor ; motor nerves arising from the floor of the 

 mesencephalon, and passing to four of the eye-muscles. 



IV. Pathetic or trochlear ; motor nerves arising from the floor 

 of the mesencephalon, and passing to the superior oblique eye- 

 muscle. 



