220 MANUAL OF ANIMAL BIOLOGY 



light-receiving elements, and is designated as the blind spot. 

 Just to one side of the blind spot is the region of the retina 

 (fovea centralis) where the vision is most acute. (W. f. 150.) 



Having now considered the three separate layers which form 

 the wall of the eyeball, mention should next be made of an impor- 

 tant accessory structure, the crystalline lens, which lies an- 

 teriorly. The lens serves to focus the light rays on the retina. 

 It is enclosed in a membrane to which the ciliary muscles, located 

 near the base of the iris, are attached. These muscles act in such 

 a way as to alter the convexity of the lens so that either near or 

 far objects may be brought to a focus. The small cavity of the 

 eyeball lying between the iris and the lens is known as the pos- 

 terior chamber and the larger cavity lying between the iris and 

 the cornea is known as the anterior chamber. Both of these 

 cavities are filled with a watery, transparent fluid, known as the 

 aqueous humor. Back of the lens is the vitreous chamber 

 which is the largest chamber of the eye, and contains a semisolid, 

 transparent substance, the vitreous humor. 



V. FUNCTIONS OF THE NERVOUS SYSTEM 



In considering the functions of the various parts of the nervous 

 system it is to be noted that the brain is regarded as the main 

 directing center of the body. In it myriads of adjustments are 

 consummated which are necessary in arranging for the correct 

 responses to all types of stimuli. In a general way, it can be stated 

 that the spinal cord is dominant to the peripheral regions of the 

 nervous system ; that the brain is dominant to the spinal cord, 

 and that the cerebral hemispheres at the anterior end of the 

 brain are the chief dominating and directing agencies of the nervous 

 system as a whole and therefore of the entire body. Furthermore, 

 if the Frog can be said to possess the power of purposive thinking, 

 or intelligence, it is located in the cerebral hemispheres. 



However, experimental work involving the complete removal of 

 the cerebral hemispheres of the Frog has shown that such an animal 

 can continue to live and carry on most of the functions in a 

 normal manner except that a certain amount of spontaneity is 

 lacking. Although it is known that the chief coordinating centers 

 of the Frog's brain lie in the optic lobes, it can be shown experi- 

 mentally that the brain may be completely extirpated as far back 

 as the medulla, and the animal will recover and be able to carry 



