Where is the heart and where is the brain?
Even a zoologist like myself can be puzzled at times by claims some people make. Said a colleague of mine to me the other day for instance: “Why don’t you write about the heart in the head of the giraffe some day?”
Well, as far as I knew the place of the heart of the giraffe was in its chest. That is where I would go looking for it, and indeed, that’s where it is. But my colleague’s suggestion did uncover something exciting. Unlike other mammals, only in the head of the giraffe just below the brain lies a complex tissue of distensible arteries (the so-called “rete”), which could help pumping up blood 3 to 4 metres high into the head, where of course the giraffe’s brain would need nutrients and oxygen delivered to it by the blood. Although some scientists favour a role as a temperature regulator for the organ in question, one wonders of course, why such a structure should occur in so well-developed a condition only in the giraffe and not any other savannah loving animal.
Also, it is an established fact that arteries by themselves (and not just the heart) undergo regular contractions to propel the blood in them forward. Flaps, similar to but much smaller than those inside the heart metres away in the chest are present in the arteries and prevent the blood from obeying gravity and flowing back. It probably goes too far to call the structure in question a “heart in the head”, but functionally it certainly assists and performs actions akin to those that occur in the proper heart in the giraffe’s chest, which without the rete’s assistance would have to be extraordinarily powerful to pump blood 3 or even 4m high against gravity.
But turning now our attention to the massive nerve ganglia in the dinosaur’s tail base, we may feel justified to be even more doubtful about the correct semantics. Brains, by definition, are nerve ganglia in the head, but some herbivorous dinosaurs like Stegosaurus had tiny brains no bigger than a walnut and even those of the largest dinosaurs are thought to have weighed only about 1/3 of a human brain. The hip ganglion at the base of the tail by contrast was 20 times as large as the brain in the head and with its assemblage of neurons must have been able to sample, sort, and integrate stimuli and “take charge” of decisions involving movements of the massive tail and perhaps the legs – the small brain in the head, after all, was metres away and signals to and from it could have taken seconds, far too long a period for decisive action.
But what about the tinier forms of life? They are weirder still, for the earthworms possesses dozens of hearts (a pair in each segment), the horseshoe crab’s heart beats above its mouth, a crayfish can sense light with its ganglion in the tail and the butterfly Papillio xuthus possesses photoreceotor on its genitals! But more about all that in a future essay.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
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