supported by a dorsal suspensory ligament. In the sharks 

 the lens is pulled forward to focus near objects by a small 

 ventral muscle in the position of the ciliary body. Amphib- 

 ians are generally like the shark; the frog has both dorsal 

 and ventral muscles for moving the lens forward. 



In reptiles and birds accommodation is peculiar in that 

 the muscles of the ciliary body are only radially disposed 

 and appear to act, in conjunction with the scleral ring of 

 bones, in changing the shape of the eyeball and thus its ac- 

 commodation. In the bird the ciliary muscles are cross 

 striated and are divided into a main portion, Crampton's 

 muscle, extending from the margin of the cornea back to 

 the middle of the inner surface of the sclerotic ring of bones, 

 and into Briicke's muscle, arising outside the Crampton's 

 fibers and extending back to the outer rim of the fibrous 

 ciliary zone. Contraction of Briicke's fibers releases the ten- 

 sion on the ciliary body (replaces circular muscles of mam- 

 mal), while contraction of Crampton's muscle acting on the 

 sclera at either end changes the shape of the eyeball, thus 

 increasing the focal distance behind the lens, moving the 

 lens forward, and causing the cornea to be more rounded 

 (forced out medially and drawn in marginally). These changes 

 focus near objects. In most birds the lens is firm and 

 inelastic, and it is enclosed marginally by a soft annular 

 ring of tissue. There is no way that the shape of the lens can 

 be changed by action of the ciliary muscles. In the cormo- 

 rant the lens is soft and elastic, and its shape is altered by 

 constriction of the iris. This constriction supplements the ac- 

 commodation effects of the ciliary muscles. 



The eye of the bird has an elaborate pecten, a comb- 

 shaped vascular fold of tissue, extending into the vitreous 

 body from the area of the chorioid fissure. This appears to 

 help in detection of moving objects and in orientation in ref- 

 erence to the sun. The retina also has one or two foveas for 

 detailed vision. A fovea and conical pecten are present in 

 some lizards. 



The variations in accommodation suggest that no intrinsic 

 device was present in the primitive eye. In some specialized 

 forms the eye may be degenerate or lost. In the process the 

 eye muscles with their nerves disappear first and then finally 

 the eyeball itself 



Median eyes 



It has been assumed that there are anterior and posterior 

 "epiphyseal" outgrowths of the diencephalic roof with eye- 

 like or glandular tips. In the lamprey the sensory tip of the 

 dorsal, ("posterior") vesicle, the pineal, has a nerve which 

 enters the posterior commissure; the lower ("anterior") 

 vesicle, the parapineal or parietal, has a nerve which enters 

 the left habenula (Figure 13-15). In the Austrahan lamprey 

 Geotria, the vesicles are side by side and the nerves enter the 

 right and left habenulae. In the Australian species Mordacia 

 mordax. the parapineal is missing. Both vesicles are lacking 

 in the myxinid. 



The embryology of these vesicles in the lamprey shows 

 that the pineal arises in association with the right habenu- 

 lar ganglion (10-mm stage); its connection with the poste- 

 rior commissure at the midline is secondary. The parapineal 

 has a similar relationship to the left habenula, which it re- 

 tains. 



In teleosts the nerve of the sensory organ enters the pos- 

 terior commissure, and therefore the vesicle is assumed to 

 be a posterior one. This is supported by the observation 

 that m the course of development an anterior diverticulum 

 appears briefly. 



In the Anura there is a "pineal eye" (Figure 13-7), whose 

 nerve enters the posterior commissure. In the salamander 

 an eye is lacking. 



Right and left nerves for the "parietal eye" were observed 

 in three embryos of Iguana 18 days old— the left nerve was 

 smaller than the right in two cases. In this genus the organ is 

 ordinarily served by the nerve of the right habenula. Iguana 

 and other lizards have a "pineal" organ whose nerve goes to 

 the posterior commissure. Both structures appear to arise from 

 a single diverticulum. 



Contrasting parietal and pineal organs suggests a fiinda- 

 mental gap between living amphibians (and fishes) and the 

 amniotes. Such a gap is not indicated by other anatomical 

 features. In the embryology of each of these groups, an 

 epiphyseal or pineal evagination appears which in the fi-og 

 or reptile divides into an eye and an epiphyseal sac. The 

 difference lies in the courses of the nerves. The association 

 of "parietal eye" and right habenula in the lizard suggests 

 that this is in fact only a pineal organ. The association of 

 the nerve of this organ with the posterior commissure in the 

 frog is probably a secondary relationship, as in the lamprey. 

 The complicated explanations usually resorted to can be 

 abandoned in favor of a more simple one, i.e. that there is 

 but a single diverticulum divided into right and left halves 

 in the lamprey, or, in the frog or lizard, into distal and 

 proximal divisions from which nerves can grow along several 

 paths. The epiphysis may not produce an eye as in the sala- 

 mander, lungfish, or mammal. 



General observations 



The origin of the vertebrate eye is shrouded in antiquity, 

 but it evidently arose within this group and is not a part of 

 the chordate heritage. In the cephalochordate, Amphioxus, 

 there are only pigment-enclosed photosensory cells in the 

 wall of the nerve tube through most of the length of that 

 tube. It seems probable that the paired eyes arose as diver- 

 ticula from the brain wall in the fashion observed in the 

 embryo. These outgrowths functioned to bring the photo- 

 sensory cells concentrated on their outer aspect nearer to 

 the surface of the head. This need arose as the animal in- 

 creased in size and pigmentation and as the cranial skeleton 

 developed. The lens was induced by the eye diverticulum 



SENSORY ORGANS 



405 



