Eye Eye, Dissection Club!

Sheep eyes are disappointingly small, unlike their testicles which we dissected last week.  If you can get hold of cow eyes or bull eyes, they are much more rewarding.  

We trimmed the muscle and fat off initially.   Then inspected the outside of the eye; the clear front (cornea), the white (sclera) and the optic nerve, which is like a nodule sticking out of the back.

The sclera of the eye is surprisingly tough. We really had to use our scissors with a lot of force to cut through this connective tissue.

We were able to remove the dark circle of tissue which is the iris and behind this was the jelly like lens. The humours (liquid) oozed out and became quite inky and messy.

Finally we turned the eyeball inside out to observe the shiny reflective tapetum lucidum, which looked like a pool of black oil with rainbows.

Dissection Club: Sheep Testicles

We dissected sheep testicles today. They were lovely specimens and an interesting tissue to dissect, being so dense.  

We first looked at the outer appearance and noted the shiny outer testis.  Exiting at the upper end is the epididymis head. The tail of the epididymis leading to the vas deferens or sperm tube is pressed against the ball sac all the way down then up again to the top.  Here we have cut through the thin membrane that attaches it to the testis and released it. We can see the associated blood vessels.

We cut into the testis and removed the outer tunica like a bag.  It was a very shiny, smooth and tough tissue with no elasticity.

Finally we cut into the testicular tissue.  When we divided it we could see the lobes or septate nature of the seminiferous tubules.

Caesarian Sections influencing Evolution

Birth weight is always the classic example of Stabilising Selection in Evolutionary Biology.  Basically, humans have difficulty giving birth; there is a trade off between large human heads and small fused pelvises required for bipedalism and human babies get stuck in the birth canal.  

Selection operates on survival of the fittest, or death of not fit.  Those babies that are too small may be born easily but are so underdeveloped they are unlikely to survive. Those babies that are too big may be well developed but they get stuck during birth, and are unlikely to survive either by being starved of oxygen or fatally haemorrhaging their mother.

So the perfect size is in the middle and selection makes sure that these individuals survive, reproduce and put their genes into the next generation.

This is an interesting article that shows that interventions in birth, ie Caesarian sections, is causing directional selection, not stabilising, and birth weight is increasing.

http://www.bbc.co.uk/news/science-environment-38210837