576 VERTEBRATE LIFE AND ORGANIZATION 



receptive cells embecUled in the skin, on the tongue, or in other parts 

 ol the body (I'ig. 29.1). In the late nineteenth century, von Frey cor- 

 related specific receptors with specific sensations. There is no doubt 

 that we discriminate between modalities of sensation, and most biolo- 

 gists have accepted von Frey's conclusions, but recently (1954) Weddell 

 and others have cpiestioned von Frey's specific correlations, at least as 

 regards the modalities oi cutaneous sensation. Other sense organs, such 

 as the eye and ear, are complex aggregations of receptor cells and 

 associated tissues. 



243. The Eye 



Ancestral vertebrates had eyes of two types— a median eye on the 

 top of the head, which probably distinguished only between light and 

 dark, and a pair of image-forming eyes on the sides of the head. 

 Cyclostomes and a few reptiles retain a functional median eye, but in 

 most groups it has become a small vestigial organ, the pineal body, 

 attached to the top of the brain. The mammalian pineal body is a 

 small, glandlike organ that has been suspected of being an endocrine 

 organ, largely because it has no other known function. There is, how- 

 ever, no clear evidence of this. Neither its removal nor the injection 

 of extracts of pineal glands has a reproducible effect on experimental 

 animals. 



Structure of the Mammalian Eye. Although the lateral, image- 

 forming eyes of different groups of vertebrates vary in their adaptation 

 for seeing beneath water, in the air, and under varying light intensities, 

 all are alike in their major features. Those of mammals may be taken 

 as an example. Each eyeball is an oval-shaped organ constructed on the 

 principles of a simple camera (Fig. 29.2 A). It has a small opening at 

 the front, the pupil, through which light enters, a lens, which brings 

 the images of objects into sharp focus, and a light-sensitive retina, 

 which is analogous to the film. 



The wall of the eyeball is composed of three layers of tissue. The 

 outermost one is a dense, fibrous connective tissue that gives strength 

 to the wall. Most of this layer is opaque and is known as the sclera, 

 but its anterior portion, through which light passes, is clear and is 

 called the cornea. The surface of the cornea is covered with a layer of 

 stratified epithelium, the conjunctiva, which is continuous with the 

 epidermis. 



The next layer of the eyeball wall is a darkly pigmented and 

 very vascular choroid coat. Its pigmentation absorbs light rays, thereby 

 reducing internal reflections that might blur the image, and its 

 vessels nourish the retina. The anterior portion of the choroid coat, 

 together with a nonsensitive portion of the retina, extends in front 

 of the lens and forms the iris-an opaque disc with the pupil in its 

 center. The iris prevents the light from entering the eye except through 

 the center of the lens, which is optically the most efficient part. The 

 amount of light entering the eye is controlled by circularly and radially 

 arranged smooth muscles in the iris that constrict or dilate the pupil. In 



