Dissection : A Tour of the Eye

The eye has been beautifully and carefully designed for its function; this post will explore the anatomy of the eye (accompanied with images from a dissection).

Picture this: sitting before you on the lab worktop is an eye which gazes into yours as you stare with fascination. You pick it up (gingerly since this is your first dissection and you’re accustomed to seeing eyes in the context of a human face) and notice that it is cool to touch, chilling your fingers through the nitrile gloves.

The human eye is around 24 mm in diameter and roughly spherical, slightly reminding you of a marble. Observe the front of the eye where the cornea, sclera, and fatty tissue can be identified. Turning the eye reveals the extrinsic muscle bundles (a human eye would have 6, however, since you are dissecting a sheep eye, you can only see 4) and the optic nerve.

Place the eye in a dissection pan and rotate it so that the cornea is pointing to the left and the optic nerve is on your right. Using a mounting needle, create a small puncture in the sclera midway between the cornea and the optic nerve – with the puncture as a starting point, you can now use the dissecting scissors to cut the eye into two hemispheres.

As you do so, you notice just how tough the sclera is. You also feel a strange fluid oozing out of the incision and trickling between your fingers; this is the vitreous humour. Along with the aqueous humour (found behind the cornea) it helps maintain the shape of the eye.

Now that you have separated the two hemispheres, you can easily identify the iridescent tapetum lucidum, the dark choroid, and the transparent retina which line the posterior of the eye – with forceps, try peeling away these layers for closer observation.

The retina is a thin layer of tissue that is responsible for converting light into neural signals for visual recognition by the brain. It has layers of photoreceptor cells that detect the colour and intensity of the light that hits them. The choroid contains an extensive network of blood vessels to support the retina. Its dark colour absorbs light so that it is not reflected around the eye.

The bluish, glittering tapetum lucidum (not present in the human eye) reflects light on to the retina and helps with night vision as it can reflect light even at very low intensities. Cats are famously known for the way their eyes shine eerily in the dark; they, too, have a tapetum lucidum layer which helps them detect prey even on the darkest of nights.

Posterior hemisphere of eye: retina, choroid, tapetum lucidum & sclera

The front hemisphere of the eye contains the lens, cilliary body, and suspensory ligaments. You carefully pry the lens from the eye.

You are initially slightly surprised by the fact that the lens is cloudy; this cloudiness is a cataract and is normal in ageing and dead specimens. In a healthy and alive creature, the lens is completely transparent. The lens’ function is to focus light onto the retina and (with the help of the cilliary body) can change its surface and shape to adapt to seeing object that are very near or very far. Gently place the lens on some newspaper and you can directly observe its magnifying capabilities.

 

Now that the lens is removed, an opening reveals itself: the pupil. Found in the centre of the iris, the pupil allows light to enter the eye and can change shape to suit the environment (with the help of two muscle layers of the iris). In the dark, the pupil dilates to maximise the light entering the eye. In intense light, however, the pupil constricts to prevent damage to the delicate photosensitive cells of the retina.

Front hemisphere of eye: iris, pupil, cornea & sclera (lens removed)

In humans, this variation in pupil size often goes unnoticed, but gaze into the eyes of the cat and you cannot possibly miss this protective mechanism of the iris. Cats’ pupils are roundest at daybreak and sunset to maximise vision at these fairly low light levels yet they reduce to a needle-like slit at midday.

This concludes your tour of the eye; you gently place the remnants of the sheep’s eye back onto the dissection pan.

 

I do hope this virtual dissection has helped you to gain a newfound admiration for this extremely well-designed organ.

Of the Kingdom of the Dead

[Warning: due to the nature of this article, there will be graphic images depicting blood]

When you see the term ‘apex predator’, what comes to mind?

Perhaps the more egoistic, yet accurate, response is ‘me’. Humans are highly skilled predators with no natural threat from any other species and that is largely due to our intelligence and ability to form complex social groups. We may not be physically designed to be the best at killing but we have a unique advantage: we kill differently. We have created sophisticated technology to contain, control, and kill animals, we specifically exploit weak and naïve prey, and we even cooperate with other species (such as dogs) to hunt.

Perhaps it is out of modesty (or mild horror at our species’ innate success in killing), but the first creature to come to mind when I think of apex predators is the majestic orca.

 

 

Often referred to as the ‘killer whale’, these cetaceans are clearly not prey; however, this term is a source of misconceptions. Firstly, ‘killer whales’ aren’t even really whales – they are actually dolphins. Furthermore, they are not as ruthlessly violent as the name might suggest and, in fact, there are no confirmed reports of an orca ever preying on or attacking people in their natural habitat – which is surprising when you factor in how poorly humans have treated them. Even the scientific name for this noble beast (Orcinus orca) has strong connotations of murder; the genus name directly translates to ‘of the kingdom of the dead’ and orca is derived from the Latin Orcus (god of the underworld and tormentor of the sinful). A rather harsh name for a species that never hunted us…

That is not to say that orcas do not kill. Orcas are the natural predators of squid, shark, otters, sea lions, penguins, birds, octopi, dolphins, and many other creatures. Their key to success in hunting is two-fold. In part, their success can be attributed to their remarkable design. I have forever been fascinated by the elegant form of the orca; it is an amazingly well-adapted cetacean. Its immensely powerful hypaxial muscles allow it to wield its tail to paralyse prey whilst the streamlined (almost bullet-shaped) body enhances its hydrodynamic movement. But neither these adaptations nor its sharp teeth are what make orcas so lethal.

The killer whale’s weapon of choice is its brain.

Pods of orcas have developed innovative hunting strategies which are passed down through multiple generations and which vary depending on the region and prey. Here are four of their best game plans.

 

Ambush on the Shore. You can’t blame sea lions in the Atlantic to let their guard down once they finally reach the coast of South America- it’s natural for them to assume that their oceanic predators can’t touch them on land. Unfortunately, this is a severe underestimation of the lengths killer whales will go to in order to get their favourite meal. For many of these careless creatures, the façade of security shatters as orcas charge at the shore and beach themselves in an attempt to snatch up a sea lion with its sharp jaws.

 

A Crippling Blow. Even one of the most popular predators and the most vicious of its kind, the great white shark, is not immune to it only natural enemy – the orca. But hunting sharks presents a major threat to the orca as, when it comes to self-defence, great whites have a far superior design to the orca’s typical prey. Rows of glistening, razor-sharp serrated teeth can tear even the thickest whale hide. Orcas must level the playing field if they want to hunt sharks: the killer whale wields its heavy tail, raising it high into the air and crashing it down on the shark’s head before flipping the disoriented victim over with its deft flukes. This complex move forces the shark into a state of “tonic immobility”. Now paralysed, the shark cannot protect itself against its predator – making it a more vulnerable, easy prey.

Orca vs Great White

 

Carousel Feeding. Orcas don’t just pick on creatures their own size; many pods also hunt smaller fish such as mackerel. Hunting smaller creatures does have a major drawback as a few small fish isn’t even a light snack for an orca- in order for the hunt to be worthwhile, they must cause as much carnage as possible. For this reason, orcas work together in small pods to gather hundreds of fish into a tight shoal (a ‘bait ball’) and, when a significant amount of prey is collected in one area, an orca will strike at the bait ball with its tail. The force of this strike stuns and kills the fish – preventing any escapes and allowing the orcas to feed on massive amounts of fish with minimal effort.

Creating a ‘bait ball’

 

Blowhole blocking. Orcas don’t limit themselves to smaller prey either. They are known to orchestrate persistent attacks on enormous cetaceans such as grey, humpback and even blue whales. With a target that can be multiple times larger than themselves, orca pods must first weaken their prey by ramming, biting, and pulling on the whale’s pectoral fins- gradually sapping its strength. Then they launch themselves onto the whale’s back and entirely block its blowhole- suffocating their victim. Eventually, drained of energy and asphyxiated, the exhausted giant drowns.

Orca vs Minke Whale (Credit: National Geographic)

 

These modes of attack, carefully tailored to the prey and situation, reveal the sheer mental capacity that sets the orca apart from the plethora of deadly creatures from the depths of the ocean.

A ruthless, calculating mind is what makes the orca the true predator of predators.

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The featured illustration of this post is a charcoal and pencil sketch of potential anatomy drawings of the orca; I created these sketches in preparation for the Nancy Rothwell Award competition.