ANATOMY OF THE FROG 65 



the cavity, reducing it to a crescentic slit in its lower portion. 

 The thickened prominences are known as the corpora striata. 



Ten pairs of cranial nerves are given off from the brain. 



The first pair springs from the anterior end of the 

 rhinencephala, and passes straight to the olfactory chambers ; 

 these are the olfactory nerves. The second or optic pair 

 arises from the sides of the brain beneath the optic lobes. 

 Each nerve starts as a broad band of fibres which runs forward 

 and downward to meet its fellow of the opposite side on the 

 under surface of the thalamencephalon just in front of the 

 infundibulum. Here most if not all of the fibres of the optic 

 nerves cross over to the opposite side, the point of their 

 decussation being called the optic chiasma. From the chiasma 

 each nerve runs outwards through a foramen in the cranial 

 wall, and passes into the eyeball. 



The third, fourth, and sixth pairs of cranial nerves are dis- 

 tributed to the muscles of the eye. 



The eyeball is moved by six muscles passing from its equator 

 to the walls of the orbit. Four of these, attached close 

 together to the inner posterior angle of the orbit, are known 

 as the recti muscles, and are attached respectively to the 

 upper (rectus superior), lower (rectus inferior), posterior 

 (rectus posterior), and anterior (rectus anterior) sides of the 

 eyeball. In addition to these a muscle arising from the 

 anterior inner angle of the orbit passes obliquely backwards, 

 and is inserted on the lower surface of the eyeball (inferior 

 oblique), and another (the superior oblique), arising close to 

 the origin of the inferior oblique, passes obliquely upwards and 

 backwards and is inserted on the upper surface of the eyeball. 

 These muscles occur in all craniate vertebrates ; the frog has in 

 addition a musculus retractor bulbi, which partly surrounds 

 the optic nerve, and lies within the cone formed by the four 

 recti muscles. The third nerve, called the motor oculi, 

 supplies the recti superior, inferior, and anterior, and the 

 obliquus inferior. The fourth nerve supplies the superior 

 oblique, and is known as the pathetic or trochlear nerve. The 

 sixth, or abducens nerve, passes to the posterior rectus and gives 

 off a branch to the retractor bulbi. The third nerve rises from 

 the floor of the mid-brain near the median line, the sixth pair 

 from the ventral surface of the medulla, and also close to the 

 median line ; but the fourth pair differs from all the other cranial 



