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The jugal bone is located on either side of the skull in the circumorbital region. It is the origin of several masticatory muscles in the skull. [1] The jugal and lacrimal bones are the only two remaining from the ancestral circumorbital series: the prefrontal, postfrontal, postorbital, jugal, and lacrimal bones. [2]
Modern examples of tetrapods without a quadratojugal include salamanders, mammals, birds, and squamates (lizards and snakes). [3] In tetrapods with a quadratojugal bone, it often forms a portion of the jaw joint. Developmentally, the quadratojugal bone is a dermal bone in the temporal series, forming the original braincase.
The acromion is a bony ridge on the outer distal end of the scapula that functions in providing an attachment for the clavicle. [2] [3] Nodosaurids develop a pronounced spur known as the pseudoacromion, which probably formed an attachment site for the Musculus scapulohumeralis anterior, and therefore is analogous with the acromion of mammals. [4]
Image of base of the skull with several of the foramina labeled. The human skull has numerous openings , through which cranial nerves, arteries, veins, and other structures pass. These foramina vary in size and number, with age. [1] [2]
It is composed of 270 bones at the time of birth, [2] but later decreases to 206: 80 bones in the axial skeleton and 126 bones in the appendicular skeleton. 172 of 206 bones are part of a pair and the remaining 34 are unpaired. [3] Many small accessory bones, such as sesamoid bones, are not included in this.
The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. [1] In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. [2] The bone forms an ancestral component of the dermal roof and is typically thin compared to other skull bones. [3]
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The axial skeleton (80 bones) is formed by the vertebral column (32–34 bones; the number of the vertebrae differs from human to human as the lower 2 parts, sacral and coccygeal bone may vary in length), a part of the rib cage (12 pairs of ribs and the sternum), and the skull (22 bones and 7 associated bones).