2o8 PHYLUM CHORDATA 



range, and fishes can see only those objects which are close to 

 them. An eye with a spherical lens is shortsighted. 



The inner coating of the eye is the retina. Between it and the 

 sclerotic coat is the choroid coat, which is black on its inner sur- 

 face and lies just beneath the retina, and silvery on its outer sur- 

 face, just within the sclera. The portion of the choroid which 

 extends over the front of the eye is the iris, the central opening 

 of which is the pupil ; delicate muscles in the iris control the size 

 of the pupil. 



Note the blind spot, the point where the optic nerve enters the 

 eye. From near this spot a slender projection of the choroid, 

 called the falciform process, extends through the retina to the 

 side of the lens, to which it is attached. 



The two large chambers of the eye are the one between the 

 iris and the cornea, which is filled with the watery aqueous humor, 

 and that between the retina and the lens, which is filled with the 

 jeliylike vitreous humor. 



Exercise 10. Draw a diagram representing the structure of the eye. 



The Ear. The auditory organ consists of the membranous 

 labyrinth alone, which is embedded in the cranium back of the 

 eye ; no external opening exists. To dissect it out is a difficult 

 task, which will not be done in this dissection. 



The Brain. If the brain has been kept in strong formalin, as 

 directed, it will have been hardened and will be in good condition 

 for dissection ; if, however, its condition is for any reason no 

 longer suitable, a fresh animal must be taken. 



Remove all the skin and the thick muscles from the head and 

 the high-arched dorsal portion of the trunk just back of it. With 

 a strong scalpel or scissors cut away the roof of the skull and the 

 dorsal wall of the neural canal of the spinal column. The brain 

 does not nearly fill the cavity of the skull but is surrounded by a 

 granular, fatty tissue ; carefully remove this substance and ex- 

 pose the brain. 



Study its dorsal surface. It is made up of five divisions. The 

 first and anterior division is the cerebrum ; it consists of a pair of 

 hemispheres, at the anterior ends of which project the olfactory 



