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.

Cetacean Vascular System

The first topic I have selected for my end-of-term review is the vascular system of cetaceans. Note that in my review of pinniped thoracic cavity necropsy I covered the heart, gaining much insight into an organ I wasn't very familiar with. General vascular anatomy is an important topic to study, and I am sure there are cetacean adaptations that are worthy to note. I am consulting Cetacean Dissections (Burne, R.H.) for this review, and will follow the pattern of blood flow described in the book.

All venous blood is deoxygenated except for blood contained in the pulmonary vein (vein denotes any blood vessel whose flow is moving towards from the heart, and is not a characterization of oxygen content). This venous blood converges upon the caval veins (inferior vena cava and superior vena cava). The coronary sinus is located within the vicinity of the entrance of the inferior vena cava into the right atrium, and this collection of vessels drains blood from the myocardium into the right atrium. From the right atrium blood is shuttled through the tricuspid valve, and so forth into the rest of the heart (already detailed in seal heart post).

The retia mirabilia (wonderful net) is a system of closely located arteries and vessels. This system is especially pronounced in cetaceans and serves a variety of purposes. Countercurrent blood flow is created by placing core-warmed arterial blood next to distally-returning venous blood. This sets up a heat gradient that lowers the rate of heat dissipation, allowing the extremities to more gradually heat up or cool down depending on the situation. Burne also hypothesizes that the network of vessels is so elaborate as to allow continuous circulation during diving, as high pressures may restrict some arteries. Scholander (1940) argues that the retia might allow blood to be shunted past the muscles, which are high in myoglobin concentration and would rob arteries of their oxygen content before their blood load arrived to vital areas. The book becomes dated when Burne explains that there is an "observed absence of a diving brachycardia (bradycardia) in the Cetacea." Noren and Cuccurullo (2004) is just one example out of many of observed bradycardia in cetaceans. Although this does not discredit Burne's hypothesis, it does show that this book is indeed from the 1950's. Retia mirabilia appear throughout the body at key areas, such as the thoracic cavity (thoracic retia mirabilia), which is found in between the heads of the ribs and which extend into the spinal canal. This connects the arteries of the chest wall with those that supply the brain.

Finally, a fascinating insight into the formation of the heart in utero is also given by Burne. While the lungs are inactive, the two sides of the heart are still connected. Communication between the right and left side is done through the foramen ovale, a partition between the two atria, thus by-passing the pulmonary artery and the lungs. This blood contains a mixture of arterial blood carried through the umbilical cord and impure blood from the fetal body. Even with the inter-atrial aperture, some blood does pass into the right ventricle and then on into the pulmonary trunk. Were it not for another by-pass between the pulmonary artery and the aorta (ductus arteriosus), this blood would reach the lungs. When the first breath is taken at birth the foramen ovale closes and becomes a slight depression (fossa ovalis), while the ductus arteriosus shrivels into a ligament (ligamentum arteriosum). During the fetal stage, the lungs are supplied with the blood necessary for growth byh bronchial arteries which extend from the aorta, and these arteries continue to nourish the lungs even after birth.

Citations:

Scholander, P.F. (1940) Experimental Investigations on the respiratory functions in Diving Mammals and Birds. Hvalradets skrifter, 22: 1-32. Oslo.

Noren, S.R. Cuccurullo, T. M. (2004) The development of diving bradycardia in bottlenose dolphins (Tursiops truncatus). Journal of Comparative Physiology. Vol. 174. 2. pp. 139-147.

Change of Plans

After meeting with my trusty advisor, Sean, it was concluded that for the remaining duration of this independent study I will be producing necropsy reports on the seals I have and will necropsy. These in-depth reports will sharpen my vocabulary, prompt valuable investigatory explanations, and give me something tangible to present in addition to this journal during my independent study evaluation. Because time is so limited and at least 2 more necropsies need to be performed, these journal entries will become less frequent and will focus only on select topics I deem necessary. These select topics are what I perceive as gaps in my anatomical knowledge and include the nervous, endocrine, and circulatory systems in cetaceans as preliminary front-runners. My current collection of books (previously noted in this journal) will be consulted alongside new finds to produce the most efficient and specialized course of study for this limited time period. Finally, although I have extensively reviewed necropsy protocols, my familiarity with the actual Human Intervention forms is lacking. Writing these necropsy reports will undoubtedly help with accustoming me with these forms, but direct studying will also occur.

There's a lot left to do, so wish me luck!

Cetacean Skeletal System

Cetaceans trace their evolutionary history back to the archicetes, and even further back to a common ancestor with the Artiodactyla (even-toed Ungulates such as deer, camels and cattle). Formerly land mammals, whales and porpoises have gone through extensive morphological and physiological changes to adapt to their aquatic environment. Looking at their fusiform shape, complete with (in many cases) a dorsal fin, it is easy to see why these mammals were once thought of as fish. Water imposes the same environmental constraints upon the fish and dolphin, hence they have convergently evolved to the morphology that is best suited for aquatic locomotion. The skeletons of cetaceans are so striking because they are so strikingly different from land-based organisms, yet distinctly mammalian. It is as if every possible mammalian bone has been morphed, while still remaining identifiable.

The general shape of these whales, as mentioned briefly before, is designed to offer the least resistance to movement in water. The head, which usually tapers off towards the snout, passes into the trunk without a definable neck. The dorsal fin is positioned near the center of the body in most odontocetes, occurring towards the lower third section of the back in larger whales. The dorsal fin is entirely cartilaginous, held up by strong ligamentous fibers and a fibrous core. The flukes provide primary movement, and is also absent of bone, except for the distal end of the backbone which lies between them.

Forelimbs are flattened and shaped into fin-like paddles most often referred to as flippers. The scapula (shoulder blade) is broad and flattened, and has an acromion and coracoid process which articulate from the scapular body in a flat projection. These two processes may occur in different shapes, or be absent from some species of cetaceans. An example of this is the humpback whale (Megaptera novaeangeliae) which is wanting of both the acromion and coracoid processes. Similarly, the clavicle is missing in all cetaceans.

One of the oddest structures of the cetacean skeleton is the lower flipper. Digits are still clearly demarcated and are found all within the same integument. The metacarpals and phalanges are indistinguishable from each other, but the phalanges occur more distally and can be identified in that sense. The first digit, known as the pollex, may be missing from certain species such as the blue whale (Balaenoptera musculus) resulting in a more slender fin. The phalanges are equipped with epiphysial cartilage at each end which is replaced by bone as the animal grows. The phalanges and metacarpals are attached to the wrist bones (carpalia, or carpals), which are cartilaginous in the juveniles of some species. These bones are in turn connected to the elbow joint, which does not allow much motion between the upper arm and the fore-arm, and is often fused together in elderly whales. The fore-arm is comprised of the ulna and radius, while the upper arm refers to the humerus. The humerus is a short bone whose head articulates with the scapula. The distal end of this bone is comparatively flattened and terminates in two facets to meet the fore-arm bones. These bones are also short and flattened, and reside parallel to each other. The proximal end of the ulna is known as the olecranon, and is a a hatchet-edge projection that occurs proximally.

External hind-limbs are completely absent in all cetaceans, but a vestigial pelvis remains. This pair of pelvic bones has no contact with the vertebral column, and the femur and tibia range in presence throughout species. Sperm whales (Physeter macrocephalus) are the only odontocetes to possess a femoral vestige, the rest of the toothed whales lacking all hind-leg bones.

The cranium is strongly compressed from front to back and appears globular. The rostrum is often elongated and serves as a beak in various delphinids. The seven cervical vertebrae are highly compressed, and may be fused in differing order in various species. Transverse processes are well developed in the lumbar and caudal regions, and diminish in size towards the tip of the tail. The chevron bones, which are "ventral V-shaped elements faceted to the under surfaces of contiguous vertebral bodies," articulate between the hind end of one vertebra and the fore-end of the next.

Rib count is also variable between species, 12-16 pairs the normal amount. The Southern Bottlenosed Whale (Hyperoodon planifrons) has the least number of pairs of all mammals, with only 8. The sternum is composed of several fused elements, with species-variable numbers of ribs attached. For example, the breast-bones of Mysteceti are only large enough to attach to the first pair of ribs.

Internal Examination

While browsing Thorndike's collection, I noticed the "Handbook of R.H. Burne's Cetacean Dissections." Although the book is from 1952, and costs "two pounds two shillings," there are many diagrams representing cetacean anatomy, along with muscle transects, anatomical pictures, etc. These visual aides are invaluable, especially to someone who is a novice to anatomy. Using Burne's book, I will detail the cetacean skeletal system tomorrow, focusing primarily on differences between human anatomy and that of the cetacean. First, however, I would like to wrap up WHOI's guide on cetacean necropsies, by going over the internal dissection, noting any contrasts between the already-covered pinniped necropsy.

The left flipper must be removed after the integument. This will allow for the locating of the prescapular lymph node, which should be peach/tan colored. It is relevant to mention, here, that College of the Atlantic does not perform necropsies with the same rigor and thoroughness that, say Wood's Hole, does. Due to our limited resources and education-focus, histology, virology, and other testing does not often occur. We are most adept at noticing macro-pathologies, such as conspicuous ulcers, impactions, etc. That is not to say that special care and attention is not given to these specimens, but merely that our abilities on-campus are limited and that sampling is less prevalent (although blubber was taken for sampling during the last harp beater necropsy). So although we would not usually preserve the prescapular lymph node for testing, it is still worthwhile to find it. The left scapula is removed by cutting through the connective tissue and muscle that is just under the bone while pulling the scapula down. A cracking sound should be heard during this removal.

The diaphragm is deflated before removing the rib cage. Again, there should be a sweet spot that is easy to cut through. Cut down the rib cage cranially. The cranial ribs are mostly double-headed, possessing two articulations, the first of which is cut, while the scalpel sweeps down the rib towards the vertebrae. The thyroid, tracheobronchial lymph node and thymus (only in juveniles, just like seals) can then be located. The lungs, like the liver in the abdominal cavity, occupy the majority of the thoracic cavity and are normally bright pink with a sponge-like texture. The lung can be detached from the trachea at the bifurcation which forms the bronchi. The trachea is longitudinally cut. Examination of the heart takes place exactly as it does for seals. Exposing the abdominal cavity is as easy as slicing mid-ventrally from the rib to the anus, cutting laterally along the thoracic arch. The liver is also lobular and is a deep maroon color. The spleen varies among cetacean species. Delphinids have a palm-sized and spherical spleen that is a "mottled dark purple to white color". Other cetaceans may have similar spleens, or they may be smaller or more ovoid. The spleen is located underneath the stomach towards the left side of the body. Accessory spleens may also be found. They act as smaller versions of the spleen. The pancreas is attached to the mesentery and sits in the curve of the duodenum. Procedure is the same for examining the mesentery, adrenal glands, kidneys, reproductive system and bladder in cetaceans and pinnipeds.

A thin tissue surrounds the stomach and is called the omentum. The stomach itself is made up of three compartments in most odontocetes. There is a fore stomach, a main stomach and a pyloric stomach. Each stomach should be examined by cutting along the greatest curvature, contents collected afterwards. The mucosa of each stomach is distinct. The fore stomach has squamous tissue that is usually white, while the main stomach has a stratified, dark red inside surface. The pyloric stomach is thinner, and is either pink or stained with bile. The remaining digestive system is then examined, followed by the brain and pituitary gland, with the same protocol as..you guessed it..the seal. An ear extraction can also be done, with protocol found in Ketten et al. (2007)




Citations:
Ketten, D.S., Cramer, S., Arruda, J. (2007) Procedure for the Removal, Fixation, and Preservation of Cetacean Ears. pp. 3.1-3.22

External Cetacean Necropsy

As much of WHOI's guide for pinniped necropsy is similar to that of cetaceans, I will be reviewing the latter section with more haste so that I can focus on advanced topics. This will also give me a chance to contrast seal and whale necropsies.

The external exam begins just the same as the pinniped's, by examining the carcass' condition. Before beginning, it is important to remember that stranded cetaceans are outside of their natural habitat, and thus apparent pathologies may actually be produced by the juxtaposition of a heavy, marine mammal in a foreign and inhospitable environment. Lacerations, bruising, organ compression and fluid pooling are to be expected, especially on long-dead specimens. Fresh carcasses (code 2) will have been stranded for less than 24 hours, while code 3 bodies will exhibit bloating, skin sloughing and other signs similar to that of the seal. Code 4 and 5 range from a collapsed carcass to mummified remains. Body condition is assessed by examining the dorsal side of the cetacean. Epaxial muscle of a robust animal is convex and rounded, while a thin animal will have a slightly sunken in girth. Emaciated cetaceans will exhibit concaveness and may have indentation of the nape.

Sex determination in cetaceans is slightly harder to do than for pinnipeds, in my opinion. the ventral midline will show a genital slit between the umbilicus and anus for both males and females. Positioning is the difference. Females should generally have their slit within 10 cm from their anal opening, while males have a greate distance. Short mammary slits may be found on either side of the genital slit in females, but may also be found in some males, and thus is not the best physical marker. Genital probing (using your finger, not weird alien equipment) can also be used. If your finger angles forward, the vagina has been entered, and if backwards, it is the penile opening. The penis may also be felt in this manner. Species differences may complicate this, but if all else fails, internal examination will reveal the truth.

Removing the skin & blubber: The cetacean should be positioned left-side up (as opposed to a ventral-side up in pinnipeds). Using a scalpel, a longitudinal incision should be made posterior to the blowhole and going down the dorsal midline, ending at the tail. Cutting perpendicularly, make 25 cm panels down the rest of the animal, making sure to make one panel under the axilla. Reflect the blubber away and cut through the fascia, revealing the skeletal muscle. Bruising in the skeletal muscle usually has a gelatinous texture and appears as a deep red or purple. The large epaxial muscle that powers the stroking action of the cetacean is then removed, along with any excess muscle on the backbone and ribs.

Next entry will tackle all of the internal anatomy of the cetacean.

Harp Seal Necropsy

This Friday, a recently thawed beater harp seal (Phoca groenlandica) was necropsied by me and a team of students and Allied Whale employees. The seal was hit by a car in its head, shattering large parts of its mandible, skull and maxilla. The force of the impact caused the seal to defecate (feces were found around the perianal region, although death was probably (and hopefully) instant. Fur from its front flippers was missing, suggesting that the beater had crawled for some time before reaching the road. Besides the cranial damage, the rest of the seal seemed to be in good shape. Concavity of the pelvic region suggested that the seal was slightly thin, but not emaciated. The presence of a penile opening clearly identified the seal as a male.

The necropsy began by removing the integument, Amanda Dunn and Michelle Klein accompanying me. The front flippers were then removed, but not before I spotted the prescapular lymph node (which I suppose I was overly excited to find). Amanda took the right and I took the left. We opened up the abdominal cavity next. A green fluid (bile?) was found inside the mesentery around the large intestines, but the intestines overall looked healthy. The kidneys were remarkably reniculated, and one showed damage in the form of lacerations (source unknown).

The thoracic cavity was then opened, and I am sad to say I failed at finding the sweet spot (mid articulations of the ribs), mostly likely due to the positioning of the carcass slightly on the right. The lungs were thoroughly soaked in blood, most likely post-mortem. The heart appeared fine, and the aorta was characteristically a pale beige. Due to the chaos of trying to remove the rib cage, I overlooked finding the diaphragm. We incised the trachea, which appeared healthy, and removed the head from the body. The extent of damage to the head was profound. Bits of skull and maxilla were resting on the tongue, and the skin of the head easily slid off the bone. The brain was clearly impacted from the force of the bones snapping back dorsally, and the teeth were sunken into the gums.

After we disposed of the carcass, everyone scrubbed-out and went home. Most of the blood had been drained before the necropsy began, both in the bag it was held in and at the accident site, so this was probably one of the cleanest necropsy's I will ever witness. I was grateful to have a code 2 body to work with, and even more appreciative of the help I received. Thank you to Spencer, Leah, Rosie, Jessica, Michelle, Amanda and especially to Jackie for helping out! Below are some pictures (courtesy of Ms. Klein)