Question

write a short notes on Skull

Answer 1

The skull is a bony structure that forms the head in vertebrates. It supports the structures of the face and provides a protective cavityfor the brain.[1] The skull is composed of two parts: the cranium and the mandible. In the human, these two parts are the neurocraniumand the viscerocranium or facial skeleton that includes the mandible as its largest bone. The skull forms the anterior most portion of the skeleton and is a product of cephalisation—housing the brain, and several sensorystructures such as the eyes, ears, nose, and mouth. In humans these sensory structures are part of the facial skeleton.Functions of the skull include protection of the brain, fixing the distance between the eyes to allow stereoscopic vision, and fixing the position of the ears to enable sound localisation of the direction and distance of sounds. In some animals such as horned ungulates, the skull also has a defensive function by providing the mount (on the frontal bone) for the horns.

Answer 2

The skulls of the several subclasses and orders vary in the ways mentioned below. In addition to differences in openings on the side of the skull and in general shape and size, the most significant variations in reptilian skulls are those affecting movements within the skull.

As a group, reptilian skulls differ from those of early amphibians. Reptiles lack an otic notch (an indentation at the rear of the skull) and several small bones at the rear of the skull roof. The skulls of modern reptiles are also sharply set off from those of mammals in many ways, but the clearest differences occur in the lower jaw and adjacent regions. Reptiles have a number of bones in the lower jaw, only one of which, the dentary, bears teeth. Behind the dentary a small bone, the articular, forms a joint with the quadrate bone near the rear of the skull. In contrast, the lower jaw of a mammal is made up of a single bone, the dentary; the articular and quadrate have become part of the chain of little bones in the middle ear. An almost complete transition between these two very different arrangements is known from fossils of early synapsids (order Therapsida).

The dentition of most reptiles shows little specialization in a given row of teeth. A dentition that divides groups of teeth into distinctive bladelike incisors, tusklike canines, and flat-crowned molars occurs in mammals but does not occur in reptiles. Instead, the entire tooth row is usually made up of long conical teeth. Venomous snakes have one or several hollow or grooved fangs, but they have the same shape as most snake teeth. The principal differences between species lie in the number, length, and position of the teeth. Crocodiles among the living forms and dinosaurs among the extinct forms have but a single upper and a single lower tooth row. Snakes and many extinct reptilian groups have teeth on the palatal bones (vomer, palatine, pterygoid) and on the bones of the upper jaw (premaxilla, maxilla). However, only one row of teeth is present on the lower jaw.

Lizards have conical or bladelike bicuspid or tricuspid teeth. Some species have conical teeth at the front of the jaws and cuspid teeth toward the rear, but the latter are not comparable to the molars of mammals in either form or function. (They are neither flat-crowned nor used to grind food.) Turtles, except for the earliest extinct species, lack teeth. Instead, they have upper and lower horny plates that serve to bite off chunks of food.