1. Axial Skeleton Chapter 8

2. Introduction The notochord and vertebral column define the long axis of the body, offer sites for muscle attachment, prevent telescoping, and support body weight. The notochord is a long, continuous rod of fibrous connective tissue wrapping a fluid filled core. The vertebral column consists of a repeating series of cartilaginous or bony elements. – The vertebral column debuts early with evidence of segmental blocks along a notochord.

3. Vertebrae The first parts of the vertebrae to appear were the dorsal (neural) and ventral (hemal) arches that rested upon the notochord. – The neural arches protect the neural tube – The hemal arches protect the blood vessels. The next part to evolve was the two centra (the intercentrum and pleurocentrum) – The centra serve to anchor and support the arches

4. At one time, the vertebral type was used to define tetrapod taxa. However, problems became evident with this theory: – Many early tetrapods evolved from aquatic ancestors, and their vertebrae became modified for life on land – Many of these secondarily returned to the aquatic environment and their vertebrae resumed their more primitive shape Thus, the morphologically similar types of vertebrae represent multiple evolutionary trends, evidence of functional convergence, not close phylogeny.

5. Centra The centra represent the body of the vertebra. Among vertebrates there is great diversity in the structure of the centra. – They may be absent: Aspondyly – Singular: Monospondyly – Or Doubled: Dispondyly

6. Centra are linked successively into a chain of vertebrae, the axial column. The shapes of the surfaces at the articulations between vertebrae affect the properties of the column and the way forces are distributed.

7. In both procoelous and opisthocoelous centra, the convex articular surface of one fits into the concave surface of the next to for a ball and socket joint. – This permits extensive motion in most directions without stretching the nerve cord. Flexion of acoelous or amphicoelous vertebrae causes adjacent centra to hinge on their edges. – Flexion causes a hinge like motion stretching the nerve cord. Heterocoelous centra allow great lateral and vertical flexion, but prevent rotation of the vertebral column

8. The notochord, or its adult derivatives, often run throughout and fill the concavities on the centra. The term intervertebral disk has been used to describe any pad between centra. – However, in specifically applies to a pad of fibero- cartilage with a core, the nucleus pulposes, derived from the notochord. These are found only in mammals – In other groups, the pad is referred to as the intervertebral body

9. Ribs Ribs are struts that can fuse with vertebrae or articulate with them. Ribs provide sites for muscle attachment, help suspend the body form a protective case around the organs, and can serve as accessory breathing devices.

10. In many fishes, there are two sets of ribs with each vertebral segment, a dorsal and ventral set. In tetrapods, one of these sets of ribs is lost, and the other (dorsal ribs) become the trunk ribs. – Ribs of primitive tetrapods are bicipital, having two heads that articulate with the vertebrae. – The ventral head, or capitulum, articulates with the parapophysis, a ventral process. – The dorsal head, or tuberculum, articulates with the diapophysis, on the neural arch.

11. Although ribs function in locomotion, in tetrapods they also become important in respiration. Classification of tetrapod ribs is based on the type of association with the sternum. – Ribs that meet ventrally with the sternum are true ribs. Consist of two jointed segments, the vertebral (costal) rib and the proximal (sternal) rib. – The joint between these two rib segments allows for changes in chest shape during respiration – Those that articulate with each other but not the sternum are false ribs. – And those that do not associate with anything are floating ribs.

12. In birds, cervical ribs are reduced and fuse with the vertebrae. – In the thoracic region, the first several, followed by true ribs that articulate with the sternum. – Some floating, and most true ribs bear a unicate process, which offer sites for the attachment of respiratory and shoulder muscles. In Mammals, ribs are present on all thoracic vertebrae and define this region. – Most are true ribs and they meet with the sternum through a cartilaginous rib segment. – Within the cervical and lumbar regions, remnants exist only as remnants fused with the transverse process.

13. Sternum The sternum is a midventral skeletal structure that is endochondral in origin The sternum is the site of origin for chest muscles. It also secures the ventral tips of the true ribs. – The rib cage consists of the ribs and sternal elements that enclose the viscera. The sternum may consist of a single bony plate or several elements in seriese.

14. Fishes lack a sternum. – When it first appears in tetrapods it appears to not be a phylogenetic derivative of either the ribs or pectoral girdle. In anurans, a single sternal element lies posterior to the pectoral girdle. The sternum is absent in turtles, snakes, and other limbless lizards, but common in other reptiles. In flying birds, the massive flight muscles arise from a large sternum that bears the keel. In most mammals the sternum consists of a chain of ossified elements in series.

15. Gastralia Posterior to the sternum in some vertebrates is a separately derived set of skeletal elements, called the gastralia, or abdominal ribs. Unlike the true ribs, gastralia are dermal in origin. And are restricted to the sides of the ventral body wall and do not articulate with the vertebrae.

16. Gastralia are common in lizards and crocodiles, serving as accessory attachment sites for abdominal muscles. Within turtles the plastron is a composite bony plate forming the floor of the shell. – It consists of a fused group of dermal elements, including the clavicles and gastralia.

17. Phylogeny

18. Fishes Agnathans: – Among ostracoderms the notochord is large and prominent. Vertebral elements are harder to document, due to lack of preservation of the internal skeleton. – Among living hagfishes and lamprey, the situation is similar. Hagfishes possess a prominent notochord but lack any vertebral elements Lamprey possess small, cartilaginous vertebral elements resting dorsally upon the prominent notochord.

19. Primitive Gnathostomes: – In most primitive fishes, the axial column consisted of a prominent notochord. – There is no evidence of vertebral centra, although dorsal and ventral arches were usually present. – Primitive chondrichthyans exhibit a similar pattern, with a prominent notochord with only cartilaginous neural and hemal arches representing the vertebral column. In advanced sharks these elements enlarge to become the prominent element of the body axis, although the notochord persists within the vertebrae.

20. Bony fishes: – Sturgeons and paddle fishes have unossified vertebral columns, a secondarily derived condition. – Teleosts have a completely ossified vertebral column, and the centra become more prominent, replacing the notochord Neural spines and ribs become more developed.

21. Sarcopterygians: – The notochord continues to serve as the primary axial support in this phyla. – In many species, such as the rhipidistians, vertebral elements are usually ossified and composed of three separate vertebral elements.

22. Mechanically, the axial column of fishes represents an elastic beam. – Lateral bending movements produced by the body musculature place the column in compression. Even during peak bursts of speed, the fish’s notochord, or vertebrae, experience stresses well within their capacity to resist breaking or collapse. Centra seem to function as compression members, and the stiffness that resists buckling is controlled by the degree of lateral flexure permitted by ligaments

23. Caudal skeleton and Fins In most fishes the axial skeleton continues into the tail. The tail is usually asymmetrical, with a long dorsal and small ventral lobe separated by a notch. The different types of tail are based on the deflection of the vertebral column.

24. a)Diphycercal: vertebral column extends straight into the base of the tail. b)Heterocercal: Vertebral column extends upwards into a large dorsal lobe of the fin. c)Homocercal: Small upward swing of vertebral column, but tail halves are mostly equal. d)Rounded homocercal: Same as before, fin rays form rounded shape.

25. Tetrapods: – As animals went from water to air their bodies went from a buoyant support design to one in which the body was supported between the limbs. – Changes in the axial skeleton are especially indicative of the new mechanical demands. Amphibians have a vertebral column composed of single, solid vertebrae at each segment, like their ancestors. – The characteristic mode of fish locomotion that depends on undulations of the body has been retained in modern salamanders and most reptiles. – Synchronized with these lateral body swings are limb movements that lift and plant the foot to establish points of rotation. What was mechanically new was the tendency to twist the vertebral column, placing it in torque. Several features of the designs of early tetrapod vertebrae can be seen as functional modifications that address these new demands.

26. One of the vertebral innovations of tetrapods was the zygapophyses. These bony processes reach across the vertebral joints to interlock gliding articulations. They are oriented to allow bending in the vertical and horizontal plane, but resist twisting.

27. The other new feature of the vertebrate skeleton is a delineation of the sacral region, the site of attachment of the pelvic girdle. – evidence that direct transfer of propulsive forces in the hind limb to the axial skeleton became an important part of terrestrial locomotion. Other changes that relate to extended exploration of land involve a loss of the connection between the head and pectoral girdle. – This was accompanied by a redesign of the 1 st vertebrae into a cervical vertebrae.

28. For early tetrapods, life on land meant that the lower jaw rested on the ground. – Opening the jaw required lifting the head, since the lower jaw could not move. – With removal from the pectoral girdle the head was now able to be lifted without interference from the shoulder. – This also meant that the head experienced less jarring during locomotion, a benefit to visual predators.

29. Amniotes: – Phylogenetically carry over their traits from the archosaour line, so the major centrum is the pleurocentrum. – In many reptiles and birds, and all mammals the intercentrum is lost. – In amniotes the head rotates primarily on two anterior cervical vertebrae. The first is the atlas, the second the axis Vertical and horizontal movements are controlled by the atlas, twisting by the axis.

30. In turtles, the shell into which the limbs and head retreat is a composite unit made of expanded ribs, vertebrae, and dermal bones of the integument that hold the soft tissues. Turtles are unique in that the appendicular skeleton lies within the rib cage.

31. The vertebral column of amniotes is often specialized. In snakes, additional zygapophyses provide additional support but do not restrict normal lateral bending. In birds, numerous cervical vertebrae have highly mobile, heterocoelous articulations giving bird skulls great range of motion. At the posterior end, the thoracic, lumbar, sacral, and some caudal vertebrae fuse to form a synsacrum.

32. In mammals, the vertebral column is differentiated into distinct regions. – 7 cervical vertebrae – 15-20 thoracic and lumbar vertebrae – 2-3 sacral vertebrae (humans have 5) Caudal vertebrae vary in number with differences in the size of the tail. Arches and zygapophyses diminish toward the posterior tip of the tail so that only centra remain at the tip.

33. Form and Function

34. Fluid Environment In an aquatic medium n organism does not depend on the endoskeletal framework for support. The body takes advantage of the surrounding water for support. For active, aquatic organisms 2 problems exist. 1.Drag on the body Fixed by streamlining 2.Orientation in three dimensional space Stabilized by fins

35. Terrestrial Environment Land generally presents a 2-D surface across which to maneuver. The major problem living on land is resistance to gravity. – When at rest, the body either sits on the ground or between the legs. – The legs act as an abutment that supports the body. – The vertebral column serves as a bridge between the support posts, the legs

36. The mammalian vertebral column, if viewed in engineering terms, might be represented by two suspension bridges, with the body between. – The spines and centra represent the compression members; the ligaments and muscles are tension members, and the legs act as the peirs. To carry weight any arch must maintain a bowed shape. – Between the pairs of limbs, the abdominal muscles and sternum keep the arched vetebral column from sagging

37. Design of Vertebrae

38. Direction of the Neural Spine Local mechanical forces on a spine arise from contraction of axial muscles. – Bones are weakest to tension and torsion and strongest when resisting compression. The neural spine is oriented in a way that its long axis is parallel with the resultant forces. – This means that the spine experiences these forces as compressive forces only

39. Height of the Neural Spine The height of the spine is proportional to the mechanical leverage that the muscles exert. In effect, the spines are levers and transfer forces to the centra. Increasing spine length increases the lever arm and, therefore, increases the mechanical advantage of the lever.

40. Regionalization In fishes The vertebral column is differentiated into two regions, the caudal and trunk regions. – Zygapophyses are generally absent. – Centra are unspecialized. The vertebral column basically offers attachment to swimming muscles.

41. In tetrapods, the vertebral column supports the body against gravity and receives and transmits the propulsive forces the limbs generate. – Trough the pelvic girdle, the hind limbs are directly attached to the adjacent region of the vertebral column, defining the sacral region. – The cervical region is also differentiated, allowing the skull freedom of movement independent of the body. Rapid locomotion causes the vertebral column to flex laterally on itself. – This can cause the ribs to crown. – Loss of ribs in the area of greatest flexion is the solution to this problem.

42. Five distinct regions are differentiated in mammals. – The musculature is attached to the vertebral column in complex ways, corresponding to the demands of their active lifestyle. Birds have major modifications that match the demands of flight. – Cervical vertebrae are flexibly articulated to give the head great freedom of movement and reach. – Conversely, most of the vertebrae in the middle and posterior part of the column are fused to each other and the pelvic girdle This brings rigidity to the vertebral Column and establishes a firm and stable axis for control while birds are in flight.

43. END The form and function of the vertebral column are related directly to the static and dynamic forces placed upon it. Overall, the axial skeleton and associated musculature contribute to: – bending the body, – storing elastic energy, – and transmitting useful forces for locomotion generated by the limbs or caudle fin.