Special Sensory Organs

My next entry is on what Burne labels the special sense organs. Cetacean sensory organs are remarkably different from terrestrial mammals, having undergone extensive aquatic adaptations. Most noteworthy, in my opinion, is the lack of an olfactory sense in most whales and dolphins. This degradation occurs in varying degrees; delphinids possess no olfactory perforations, while baleen whales have perforations of their mesethmoid bone (the most anterior bone of the braincase, corresponding with the cribriform (upper nasal region) plate found in mammals), although these only host simple nerves that connect to the nasal passage.

The lens of the eyes are highly convex in cetaceans. The posterior segment of the bulbus oculi (globe), which includes the back two-thirds of the eye, is relatively shallow. Both of these adaptations better suit the cetacean for aquatic vision. The margins of the cornea are thickened and the optic nerve is encased in ophthalmic rete to protect the fragile eye from general, and diving, water pressure. The tear gland (lachrymal gland) secretes a greasy substance in cetaceans, and along with the conjuctiva (exterior eye coating) which is replaced by, in some whales, a horny epithelium, protect against water damage.

The lack of external ears (pinnae) in whales is an easily noted but intriguing oddity. The internal ear is also highly adapted for aquatic life. The entrance to the ear is streamlined with the body, and only vestiges of the auricular cartilage are found in some whales underneath the integument. The tympanic membrane (eardrum) of baleen whales are quite distinct. Shaped as a long, hollow an conical structure, this membrane can be as long as 3 inches. An earwax plug extends out from the eardrum. Odontocetes have a traditional mammalian eardrum. In the tympanic cavity there are auditory ossicles that connect to each other. The ossicles occur in the following form going inwards: hammer-bone (malleus), anvil-bone (incus), and stirrup-bone (stapes). The malleus connects to the eardrum and is also fused to the tympanic bulla, while the stapes is located in the innermost wall of the middle ear (fenestra ovalis). These three ossicles form an irregular column between the tympanum and the inner ear. Tympano-periotic bones are the densest bones in the cetacean skeleton, the tympanic bulla the most prominent representative.

(Pictured: tympanic bulla of a cetacean)

The eustachian system, which connects the middle ear to the nasal passages of the pharynx is specialized in cetaceans. Baleen whales possess a sac (pterygoid sinus) that is found in the region of the pterygoid bone. These sacs (diverticula) are extremely specialized in dolphins and occupy much of the sinus region. Two diverticula are found along the roof of the mouth, reaching to an inch of the tip of the snout. In most mammals, such as humans, the eustachian system is barely even a system, comprising only a thin tube that is often closed. The specialization of this system in cetaceans, especially delphinids, is suggestive of the importance of hearing in these species for external contact. With the olfactory senses atrophied, cetaceans benefit from this additional sensory system for life functioning.

Well there you have it for special systems! This is likely my last anatomical entry; two seals and three necropsy reports need to be done (dissected and written, that is) in the next 2 weeks, and I still have to familiarize myself with human interaction sheets. I was surprised by how much anatomical jargon I was introduced to. Even simple terms like aperture and foramen were newly learned, and I am now comfortable with several systems I previously knew little about. I've also established a foundation upon which more anatomical lessons may be placed. I will post the necropsy reports once they are finished, but until then, ciao.