Quadrate bone

The os quadratum (short quadrate, sometimes also called " square leg ") is a paired cranial bones of Osteognathostomata ( bony fish, including terrestrial vertebrates), the "higher" in all vertebrate large groups, except in mammals, is the primary jaw joint. In mammals, this bone is as an anvil ( incus ) involved in the hearing bones series. Because of its anatomical position, the quadrate is central to the Schädelkinetik. In anatomical drawings, the quadrate is usually marked with the Q or q.

The upper jaw of "primitive " gnathostomes is called palatoquadrate - it go later palatine bone ( palatine ), Metapterygoid and square leg bone fish out while the piece in the cartilaginous fish is still undivided ( merged with the chimeras and lungfish with the skull ). The lower jaw ( mandible which ) always directed at the square part or quadrate ( saddle joint - for fish, from the beginning, and the only long diarthrosis ). According idealistic morphology go these two skeletal pieces back to the front gill arch of jawless fishes ( Agnatha ).

It was thought as the lower jaw forward would initially also had a joint of the maxillary symphysis. One sees now but at Acanthodii that the Palatinteil a secondary acquisition, through outgrowth is - the articulation of this " Pterygoquadrats " with the neurocranium behind the eye socket is so primitive. The upper jaw can not correspond to a Epibranchiale. In order to expand the oral cavity transverse ( crosswise) stronger ( loops, respiratory ), this upper jaw ended gelenkend then the palatine (ie without symphysis ) - the front edge of dorsal mouth took over dentulous covering bones of the nasal capsules. The former upper jaw, the tooth bases inside now developed (at the mouth) also cover bone is so - in conjunction with the rear supporting or leading Hyomandibel - for " jockstrap " and then loses the pterygiale skull - joint. The ventral ossification of the mostly triangular Suspensoriums is at Teleostei almost always triangular quadrate that due to participation in the TMJ never teeth, so no covering bones, has. Only at the trailing edge is close contact to the main reinforcing element of Suspensoriums, the Praeoperculare. Otherwise, the contact to the dorsal bones of the Suspensoriums ( the pterygoid ) because of the necessary spring effect only cartilaginous ( Palatoquadratknorpel ) - also the mandible, therefore, shows similar construction (see Meckel 's cartilage ). The Hyomandibel is " wedged " behind by the symplectic into Quad Tatum. A free movable quadrate is ( insidiator Epibulus, Petenia ) very rare in fish - otherwise, with the jockstrap is ( by the levator arcus palatini, adductor arcus palatini et hyomandibularis and the floor of mouth and hyoid muscles), mainly transversely movable ( widening and constricting the oral cavity ). There are also fish with immovable jockstrap - breathe then only with the Branchiostegalmembran ( Uranoscopidae ) or almost exclusively with the gills ( Mormyridae, Serrasalmidae etc.). From the outside Quadratum springs almost always a part of the adductor of the mandible ( jaw muscle ), but also on the inside, a part of this muscle arise ( Aω to the inside of Meckel 's cartilage ).

In the terrestrial vertebrates with a large jaw is not disturbing - the fish - jockstrap was functional ( for breathing ) unimportant. Since the gill basket is reduced in adult Lissamphibia, the skull can be shortened: the quadrate as TMJ carrier is often at the widest point of the head: at or near its posterior margin. It is stretched, columnar, somewhat movable and supported on the skull roof. With the upper jaw, it can be connected through the Quadratojugale. The Hyomandibel is reduced next to it, has lost its guiding function ( the jaw apparatus is again Autostyl as may at the earliest pine mouths ), but a new acquired: they now transmits sound waves (initially from the ground) to the ear labyrinth. " Contrary to expectations, " even the skull of the armor- headed Gymnophionen something kinetically.

In reptiles the location and function of the quadrate is not much different. In detail, sees the mobility - in the context of the zygomatic arch - but quite different from. The turtles and crocodiles have akinetic skull. The form- rich skulls of lizards, specifically the Diapsida are in different design and in varying degrees kinetically ( meso- or metakinetisch - depending on whether only the front part of the Dermatocraniums or this is completely lifted from the quadrate from the mouth unlocking: Streptostylie, explored particularly by January Versluys 1912, 1927, 1937; see Squamata ). Only with the snakes the element with the resolution of the rigid structure of the food-acquisition - system ( visceral cranium ) has again become very mobile. The large, a connecting rod movable quadrate moves in front of only the jaw apparatus ( the mandible with retraction blocking dentition, usually even jaw halves separately ) here - the other " Dermatocranium " remains unmoved. ( A distant similarity is obtained here to some deep-sea fishing (esp. Stylephorus ) and the troublemakers, but where the jockstrap or Hyomandibel works a connecting rod. ) Is moving these connecting rods from descendants of those muscles that "earlier" the jockstrap moving (Ab - and adductors ).

Os quadratum of birds

In birds, the four-sided quadrate is rotatably attached to the base of the skull. The body of the quadrate ( Corpus square ) has three extensions. The ear extension ( processus oticus square ) is for connecting to the ear bones (ossa otica ). The lower jaw extension ( processus mandibularis square ) forms the articulated connection to the mandible, more accurately articular Os. The orbital extension ( processus orbital square ) is the eye socket (orbit ) directed. In him is where the musculus protractor pterygoidei et square.

By train the musculus protractor pterygoidei et square (syn. musculus craniopterygoquadratus, sphenopterygoquadratus musculus ) is moved, the quadrate with its lower half to the front, which looks like a connecting rod attached to it move quadratojugale Os and Os also jugale forward and the maxilla push upwards. The upper jaw of the birds is by an elastic zone (a kind of pseudo joint) connected to the rest of the skull ( the bird skull is mesokinetisch ). In contrast to mammals, where only the lower jaw can be moved down to the foot- opening, birds can at the same time the upper jaw upward move ( clearly seen for example in parrots, hardly the ostrich ). The Musculus pseudotemporalis profundus (syn. quadratomandibularis musculus ) and the musculus adductor mandibulae caudalis that focus on the body of the quadrate, the quadrate turn back and move so that the upper jaw downwards ( ie in general as for the lizards and snakes). Since the rotational axis of the " pseudo- joint " is in front of the eye socket, it is called a " prokinetic skull "; at several Charadriiformes only the tip of the beak is even bent.