THE POSTCRANIAL SKELETAL





The primary function of the cranial skeleton was to protect and support the brain and associated sensory organs. In contrast, the postcranial skeleton is used less for protection but more for support of the body and for locomotion

The postcranial skeleton can be divided into two regions:

Trunk - includes the vertebral column, ribs and sternum (part of axial skeleton)
Appendicular skeleton - limbs and girdles
Notochord vs. the Vertebral Column

In early vertebrates the notochord is a non-bony skeletal support for swimming by lateral undulation

- consists of vacuolated cells surrounded by a thick fibrous sheath that maintains rigidity, but is also flexible
- ventral and parallel to the spinal cord (= dorsal hollow nerve cord) in vertebrates
- absent in hemichordates, present in the tail of most larval urochordates but lost in the adults
- present in the body and tail of cephalochordates and vertebrates
- present in the embryo of all vertebrates but, as the vertebrae form around it, it becomes constricted and in many groups, such as mammals, there is almost nothing that remains.
The vertebral column is a basic support structure that is developed from individual metameric units called vertebrae which replaces the notochord

The basic units of an individual vertebra are:

  • Vertebral/neural arch - surrounds and protects the spinal cord
  • Hemal arch - surrounds and protects the caudal artery and vein in fishes
  • Neural spine - spinous process that projects dorsally from the neural arch
  • Centrum - body of the vertebra which replaces the notochord; shape depends on the vertebrate class (Fig. 8.4, p. 277):
  • Transverse process - spinous process that projects laterally from the centrum
  • Zygaphophysis - articular processes that extend forward and backward from neural arches and help to strengthen the union between vertebrae
  • Intervertebral disks - pads derived from part of the notochord and composed of fibrocartilage and connective tissue that lie between adjacent centra that help to cushion the connection
  • In tetrapods, two general anatomical relationships occur between centra and neural arches:
  • aspidospondyly - all arch elements (intercentrum, pleurocentrum, and neural arch) remain as separate ossified elements
  • holospondyly - all vertebral elements are fused into a single piece
  • For most vertebrates (except for most fishes), regional variation in the appearance of the vertebrae are recognized (Fig. 8.2, p. 277):
  • Cervical - vertebrae of neck (not found in fishes)
  • Atlas - the first cervical vertebra of terrestrial vertebrates which articulates with the skull; nodding movements of the head occur between the atlas and the skull
  • Axis - the second cervical vertebra of mammals; rotary movements of the head occur between the atlas and axis
  • Thoracic - vertebrae of chest region, articulate with ribs
  • Lumbar - vertebrae of lower back
  • Sacral - lower vertebrae that are fused (three fused in dog and cat, five fused in human) as the sacrum
  • Caudal - vertebrae of tail (pygostyle in birds forms attachment for tail feathers)
  • Ribs and Sternum

    The ribs and sternum serve several general purposes:

    - help to strengthen the body wall and lend support to the thorax
    - protect the organs of the thoracic region (circulatory and respiratory)
    - used as the site of muscle attachment
    - in amniotes they are used to assist in breathing
    There are two general ways in which ribs can develop in different vertebrate classes: Intermuscular (dorsal) ribs - develop in the myosepta between the myomeres and attach to centra of vertebrae, between the dorsal and ventral muscle masses
    Subperitoneal (ventral) ribs - form between the ventral muscles and the lining of the coelom
    With the exception of the agnathans that have no ribs at all, fishes generally show both types of rib morphology - may be found throughout the trunk, as is the case in salamanders and reptiles, or only in the thoracic region, as is the case for mammals and birds Mammalian ribs articulate with the vertebrae at two points: - the dorsal point of articulation tuberculum
    - the ventral point of articulation capitulum
    - also articulate differently with the sternum, and can be either true ribs (directly attached to the sternum via costal cartilages - 7 pairs in human), false ribs (attached indirectly to the sternum through the costal cartilages of the other ribs - 5 pairs in human) and floating ribs (do not attach to the sternum - 2 pairs in humans).
    The sternum forms a complete enclosure of the chest region in conjunction with the ribs and is connected closely to the shoulder girdle and ribs. - possessed only by tetrapods, with the exceptions of snakes and turtles (completely lacking)
    - acts to protect the thoracic region
    - serves as a site for attachment of the pectoral limbs
    - aids in rib movements
    In birds, the sternum is completely ossified: ratite - lacking a distinctive keel or sternum for attachment of breast muscles
    carinate - having a massively enlarged sternum to support flight muscles
    In mammals, the sternum is divided into three regions (anterior to posterior): manubrium
    sternebrae (ossified bony elements)
    xiphisternum and xiphoid cartilage
    Several vertebrates, including lizards and crocodiles also posess gastralia: - ribs of dermal origin restricted to the sides of the ventral body wall
    - do not articulate with the vertebrae but act as an accessory skeletal system for muscle attachment and support for the abdomen


    Definitions

    Acoelous - centrum flat on posterior and anterior surface, characteristic of some reptiles and mammals
    Amphicoelous - centrum hollowed at anterior and posterior end, characteristic of teleosts and early reptiles
    Amphistylic - jaw is supported both by the hyomandibular and by a direct connection between the jaw and the chondrocranium; found in primitive cartilaginous fishes
    Atlas - the first cervical vertebra of terrestrial vertebrates, which articulates with the skull; nodding movements of the head occur between the atlas and the skull
    Autostylic - upper jaw (pterygoquadrate cartilage) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibular found in lungfishes, and in the tetrapod ancestors
    Axis - the second cervical vertebra of mammals; rotary movements of the head occur between the atlas and axis
    Calcified cartilage - cartilage containing deposited calcium salts. Found in the vertebrae of cartilaginous fish
    Carinate - having a massively enlarged sternum to support flight muscles
    Cartilage - a firm but elastic skeletal tissue whose matrix contains proteoglycan molecules that bind with water.
    Choanae - internal nostrils that pass from the external nares to the lungs
    Chondrocranium - anterior part of the axial skeleton that encases the special sense organs and contributes to the skeletal elements encasing the brain
    Chondrocyte - a mature cartilage cell
    Composite materials - heterogeneous materials composed of many different elements, that is generally much stronger than homogenous materials.
    Cricoid cartilage - cartilage of the larynx that develops from the hyobranchial apparatus
    Dermatocranium - consists of dermal bones that encase the chondrocranium and splanchnocranium and contribute to the braincase, jaws, and skeletal elements of the mouth (teeth)
    Elastic cartilage - cartilage containing elastin fibers that appears yellowish. Found primarily on external ear and epiglottis.
    Fibrocartilage - cartilage containing collagen fibers. Found in the intervertebral disks and pubic symphysis
    Fontanelle - temporary gaps between bones, such as those that occur during the formation and fusion of the cranial bones
    Heterodont - teeth modified for different functions
    Hyaline cartilage - cartilage with a clear translucent matrix. Found primarily on the ends of ribs and on the trachea
    Hyomandibular cartilage - dorsal part of the hyoid arch that articulates with the chondrocranium
    Hyostylic - upper jaw loses any major direct connection with the chondrocranium and the upper and lower jaws are supported solely by the hyomandibular; found in elasmobranchs and most bony fishes
    Lacunae - small cavities, such as those in bone or cartilage, that contain osteocytes or chondrocytes
    Membrane bone - superficial bones that lie in or just beneath the skin and develop from the direct deposition of bone in connective tissue
    Opisthocoelous - centrum convex on anterior surface, concave on posterior surface, characteristic of some of the vertebrae of reptiles and mammals
    Perichondrium - the connective tissue that lies on the outside of cartilaginous tissue
    Procoelous - centrum concave on anterior surface, convex on posterior surface, characteristic of some reptiles and amphibians
    Pygostyle - the fused caudal vertebrae of a bird that support the tail feathers
    Ratite - lacking a distinctive keel or sternum for attachment of breast muscles
    Sacrum - the union of two or more vertebrae and their ribs, by which the pelvis articulates with the vertebral column
    Sclerotic bones - bones that surround the orbit but do not articulate with it, and provide reinforcement for the eye
    Splanchnocranium - or visceral arches that support and move the gills and contribute to production of the jaws in gnathostomes
    Thecodont - also called homodont, or undifferentiated tooth structure
    Turbinate bones - bones of the nasal cavity that increase the surface area available for olfaction
    Wormian bones - intermediary bones, or small islands of bone that occur between sutures in the skull
    Zygaphophysis - articular processes that extend forward and backward of neural arches and help to strengthen union between vertebrae