Chapter 41 Diseases of the Special Senses
The ear has both auditory and vestibular function in the snake. In many reptiles, the ear is located in a shallow depression on the side of the head and is covered by a thin piece of tissue that is shed during ecdysis. Snakes lack an external ear and have no tympanic membrane or middle ear cavity. Although it has been the general belief that snakes have little hearing capability, they are able to hear sounds in the low-frequency range of 150 to 600 Hz.
The eye is different from that of mammals. The iris contains striated muscle and does not respond to common mydriatics. The pupil is round and relatively immobile in diurnal species and slitlike in nocturnal species. There is no consensual pupillary light reflex. Descemet’s membrane is absent from the cornea. A fusion of the lids to form a protective spectacle over the surface of the cornea allows tears to flow between the spectacle and the cornea to maintain moisture. The secretions drain into the lacrimal duct, which drains into the oral cavity. The retina is relatively avascular but contains the conus papillaris, a large vascular body that protrudes into the vitreous. Examination of the eye in reptiles is challenging.
Specialized infrared sensors called heat pits can be found in Booidea and pit vipers. The location of these sensors depends on the species. These allow the snake to navigate and find food in darkness and to sense small temperature variations.
Spectacles cover the entire surface of the cornea in the snake. These spectacles undergo changes during the shedding cycle, becoming cloudy before the shed, then clearing after shedding is completed. Retention of the spectacles is a common condition seen in pet snakes and, in many cases, is a result of low humidity in the environment (Fig. 41-1).
Figure 41-1 A retained spectacle in a snake.
(From Mader DR: Reptile medicine and surgery, ed 2, St. Louis, 2006, Saunders. Courtesy S. Barten.)