'TUBULAR' EYES; SPHERICAL LENSES 213 



locked in a close-fitting orbit and cannot be turned. Even though the 

 oculomotor muscles are present in owls, the eye of an owl cannot be 

 moved in the orbit by force. In consequence, the owls and the lowest 

 primates (e.g., Tarsius) have evolved an extraordinary rotability of the 

 head upon the axis of the body. The neck in all birds is notoriously 

 flexible — even the strictly diurnal hawks can rotate the head about 180 ; 

 but the owls can revolve theirs through 270° or more. To explore their 

 surroundings visually, the deep-sea fishes, lacking a neck, must turn the 

 whole body, or bend the trunk if they are slim enough to do so. 



Spherical Lenses — Where the eyes of small nocturnal animals have 

 remained spherical and not enlarged unreasonably, the lens is always 

 even larger in proportion than in tubular eyes. In fact, when the lens 



Fig. 84 — Tubular (miscalled 'telescopic' ) eyes. 



a, owl, Bubo sp. x 1. After Putter, b, prosimian, Galago crassicaudatus garnetti. x2.46. 

 After Franz, c, deep-sea fish, Argyropelecus sp. Redrawn from Hesse. 



swells (through evolution) in size it swells also in shape, so to say, and 

 tends toward a sphere (Fig. 71, p. 173). When it has attained this shape, 

 as in small bats, most rodents, and the rodent-like opossums, an advan- 

 tage is gained in connection with the need for voluntary eye movements 

 — the latter can be allowed to diminish or even to disappear. Part of the 

 reason for this is the absence of an area centralis, owing either to its dis- 

 appearance or to a failure to evolve one. Since there is no reason to aim 

 any particular retinal spot at the object under scrutiny, there is no reason 

 for aiming the eye at all. Largely, however, the diminution of eye move- 

 ment is due to the periscopic action of a spherical lens when associated 

 with a concentric or nearly concentric retina. Such a lens casts an image 

 which is small, but is equally good from whatever direction the object 



