Sunday 10 September 2023

What's a head? Or a neck...

Yes, it's a silly title. Isn't a head, rather obviously, that part of an animal where vision and hearing are gathered, along with their associated neural processing units, as well as air and food intakes, along with whichever specialised organs that takes? For air intake, you could read 'nose', and for 'food intake' there is the whole complex assembly of jaws, teeth, a tongue as well as an oesophagus. 

That seems right for humans and other terrestrial vertebrates, but a few moments' thought reveals that 'obviously' does not belong in this 'definition'. Fish have heads but take in water, not air, so the 'air intake' should become an 'inlet for gas exchange'. The hearing organs of many arthropods are not in their heads, and so the associated bits of brain need not be in the head either. The description supposes that the rest of the animal is distinct from the head. But in octopuses there is no distinction between head and body, with the limbs ('arms' that are not accompanied by 'legs') attached directly to the head/body unit. For spiders a similar point can be made that the head is merged with part of the body. 

I could go on, but the point is that a 'head' is not as clear an entity as you might think. A 'head' is the result of 'cephalisation', described by Wikipedia as 'an evolutionary trend in which... the mouth, sense organs, and nerve ganglia become concentrated at the front end of an animal, producing a head region.' I like 'head region': it provides the looseness we apparently need to describe what a head is. 

Furahan 'Scalates' certainly underwent cephalisation, and their heads definitely contain eyes (four of them) and ears, in terrestrial forms (also four) along with enough brain to do the heavy duty processing these organs require. But the 'intake for gas exchange' is not in the head at all. The intake for food is there, with jaws (originally six, later four) at the front of the animal, so clearly these are in the head. Well, that depends... 

Click to enlarge; copyright Gert van Dijk

A clade of Furahan Fishes ('Fishes V') developed a 'neck' in the form of a two-bone articulated connection between the body and the 'head region', allowing the latter considerable freedom of movement relative to the body. The thing is that a similar 'neck' also evolved between the 'head part' housing eyes and ears, and the mouth region. Are there then two heads, or is the entire region one head? 

The two necks, usually called the proximal (closest to the body) and distal (farthest from the body) neck, each consist of two long bones with a joint in between. The two 'heads' are the 'sensocranium' and the 'orocranium'. The image above shows a fragment of a painting showing a species of Fishes V with just that arrangement. You cannot see the joints in the necks clearly in these streamlined Fishes, but they are there. Those long neck bones form a big difference from the kind of vertebral columns we are used to, with their string of many small bones. Early Scalates never had a central string of small bones for evolution to play with; instead, they had a 'ladder'. The resulting neck movements look rather like those of a vertebrate arm or leg, with sharp angles, not at all like the curves of a lizard tail or a giraffe or sauropod neck. But that ungainly look need not be a functional handicap; despite the sticklike nature of human arms, baseball and darts players manage to land small objects with incredible precision quite a distance away. Of course, the trick is having a good brain in control.

We now only need to discuss bending of other organs in those necks, such as the oesophagus. An oesophagus relies on peristalsis making it flexible and elastic, so the structure itself should be able to withstand folding for a while. The nerves and arteries in our shoulders and elbows have to withstand bending too and usually do so fine. (Admittedly, if you spend too long in one position local pressure may pinch blood vessels, so people occasionally find that an arm or a leg, or just one nerve, has 'gone to sleep'.) If the oesophagus would be folded, it would probably be incapable of propelling food. If so, the distal neck has to be straightened between bites to allow the animal to swallow which is not a problem and looks interesting. Perhaps a different problem is that the length of the oesophagus may have to vary with neck position. If the oesophagus lies in front of or below the neck bones, as it does in humans, bending the neck backwards and upwards requires the oesophagus to lengthen. It has to be elastic anyway, so some degree of lengthening should not be a problem. 

Click to enlarge; copyright Katrina van Grauw

There is another solution though. In birds, the trachea (windpipe) and oesophagus do not commonly lie in front of the neck vertebrae, but to one side. This is not the exception but the rule, in fact, as shown by this paper. Most often the trachea and oesophagus lie toon the right side, but that can differ between individuals, with recorded examples on the trachea on one side and the oesophagus on the other. If birds bending their generally long necks, the oesophagus does not have to follow the bones. The oesophagus literally cuts corners. 

Click to enlarge; source here

The two images above show this odd anatomy very clearly, with a heron as an example. One is from the excellent work 'The unfeathered bird' by Katrina van Grauw, and the other is from the paper mentioned above. The oesophagus runs almost in a straight line when the neck is curved, which also means that the oesophagus is then much shorter than when the neck is fully extended. It has to be very elastic. The explanation for this arrangement is that the trachea and oesophagus can move so freely because they are not restrained by muscles as is the case in mammals. Some birds can swallow enormous prey (herons again) which also requires that the oesophagus has freedom of movement. 

Click to enlarge; copyright Gert van Dijk

I am revising an older image of a herd of large hexapods thundering into view. To get a better idea of their heads, I sculpted one in Zbrush. 

Click to enlarge; copyright Gert van Dijk

Click to enlarge; copyright Gert van Dijk

As you can see, I played with a lateral position of the oesophagus. The oesophagus forms a distinct bulge on the left side of the distal neck, cutting corners in bird fashion. The sensocranium sports shields and horns to impress others of its own species. When the animals start bashing one another's heads and necks, the necks should be protected too, so the middle joint of the proximal neck also bears some shields. So here is the somewhat baroque Latifrons augustus ('elevated broad-brow').