ANATOMY OF THE FROG 69 



Anteriorly the sympathetic chain makes communications with 

 the cranial nerves by a branch which passes on either side 

 from the first ganglion to the ganglion nervi vagi, where some 

 of its fibres enter the vagus, the remainder pursuing their 

 forward course to enter the Gasserian ganglion of the fifth 

 nerve. 



We may here consider two collections of nervous ganglia 

 in the heart which are connected with the vagi and the 

 sympathetic system. The two vagi pass to the sinus venosus 

 and enter a nervous ganglion situated in its wall and known 

 as Eemak's ganglion. Thence the vagi pass into the auricular 

 septum, one in its dorsal and the other in its ventral portion, 

 and run backwards to the region of the auriculo-ventricular 

 groove where they enter another collection of nerve-cells 

 known as Bidder's ganglia. From these ganglia fibres are 

 distributed to the rest of the heart. These two minute collec- 

 tions of nervous matter are of importance to physiologists. 



The eyeball of the frog is attached to the skull by muscles 

 which have already been described. The eyeball is not 

 spherical, but is flattened on its outer side, more convex on its 

 inner side. It consists of the following parts : (i) a firm outer 

 wall, the sclerotic, formed of cartilage and dense white con- 

 nective tissue ; (2) the cornea, a transparent area on the ex- 

 posed part of the eyeball, through which light is admitted into 

 its interior : it is continuous with the sclerotic ; (3) a coloured 

 curtain, the iris, lying behind the cornea, and surrounding a 

 central aperture, the pupil. The iris is provided with circular 

 and radical muscle fibres, by means of which the size of the 

 pupil can be diminished or enlarged. In the interior of the 

 eye a firm transparent spheroidal body lies behind the iris, 

 attached to its outer margin. This is the lens, and it serves to 

 focus light upon the back of the eye. Between the corner and 

 the lens is a small space, the anterior chamber of the eye, filled 

 with a watery fluid, the aqueous humour. Between the lens 

 and the back of the eye is a much larger space, the posterior 

 chamber of the eye, filled with a gelatinoid material, the 

 vitreous humour. The sclerotic is lined by a black pig- 

 mented layer called the choroid, which is continued in front 

 into the iris. Inside of this, lining the chamber of the eye, is a 

 delicate transparent membrane, the retina, on which the 

 ramifications of the optic nerve are spread. It is the sensitive 



