So, what are semicircular canals? To begin with, they are not actually bone. They are membranes. Technically when paleoanthropologists study semicircular canals they are looking at the bony labyrinth – that is the bone surrounding the membranes. In short, considering this part of my “what you can learn from bone fragments” series is somewhat misleading.
Semicircular canals develop from paired otic placodes (Note to developmental biologists: Yes I understand this is a simplistic version of the ontogeny of semicircular canals). Each placode divides into superior vestibular and inferior cochlear parts (the cochlear part winds about it’s axis towards the vestibular part and developes earlier at a faster rate). The semicircular canals emerge from the vestibular portion and display growth rate differences (the lateral canal emerges at a slower rate). In humans the cochlear portion mirrors the flexed cranial base (during growth the cranial base in humans starts out relatively unflexed and slowly flexes and the cochlea follows). They are completely developed and obtain adult size by two years of age (which is why we could talk about the shape of the semicircular canals in the Dikika find).
The semicircular canals are located in the temporal:
You can see where they are located in this picture:
So now that we know where they are let’s talk about what they are and what they do. There are two parts. The osseous labyrinth and the membranous labyrinth. The osseous labyrinth is, basically, composed of three parts. These are the vestibule, the semicircular canals and the cochlea. In this post we are primarily concerned with the osseous labyrinth, but some knowledge of the membranous portion will be helpful (see below).
There are three semicircular canals (which are superior and posterior to the vestibule): the anterior, posterior and lateral. They arise from and return to the vestibule and shortly before they reach the vestibule they expand slightly (this area is called the ampulla). They contain the semicircular ducts (which are about 1/4 the size of the canals). They also contain a substance called perilymph which helps suspend the semicircular ducts). The anterior and posterior canals are oriented at an approximately 45 degree angle (you can find more abbout this here). The vestibule is the large central area which contains the utricule and saccule. Anterior to the vestibule is the cochlea, a spiral shaped structure. The osseous labyrinth looks like this:
(Note: you will also see a structure, in the above picture, called the oval window, forensic anthropologist can use the oval window to separate Native American skulls from Caucasoid skulls).
Together the osseous and membranous labyrinths look like this:
So now that we know what they are and where they are, we are in the position to discuss what they do. In part three I will be providing an overview of what is currently known about the semicircular canals of various primates (both extant and extinct) and will discuss function then (I am hoping to have this up by Friday – definitely by Saturday).
Filed under: Bone Fragments