Lessn 11 Lessn Outline: Frm and Functin f the Axial Skeletn Reginalizatin f the Vertebral Clumn Bridging Design f Vertebrae Angle f the Neural Spines Height f Neural Spines Ribs and their Derivatives Sternum Objectives: At the end f this lessn yu shuld be able t: Discuss the majr design features f vertebrae listing the majr frces invlved in prducing change and the bilgical significance f the slutins. References: Chapter 8: 148-161 Reading fr Next Lessn: Chapter 8: 143-161 Chapter 7: 133-142
Frm and Functin Reginalizatin f the Vertebral Clumn In fishes the vertebral clumn is differentiated int tw regins, the trunk and the tail. The centra are undifferentiated reflecting the fact that the clumn is nt used fr supprt. The nly differentiatin is whether the vertebrae receive ribs r hemal arches. N mvement is pssible between the first vertebra and the skull. In tetrapds, the clumn supprts the bdy and receives and transmits the frces frm the limbs that generate lcmtin. Since diverse frces are placed n different parts f the clumn, nt surprisingly we see differentiatin f specialized regins. The first tw regins t specialize in early vertebrates are the cervical regin allwing sme freedm fr the head t turn independently f the bdy, and the sacral regin fr the attachment f the pelvic girdle and the hindlimbs. The Crani-Vertebral Junctin Flexibility between the skull and vertebral clumn was achieved by develpment f a mre mbile jint, and eliminating ribs in this area. The first vertebra, r atlas, is highly mdified in all vertebrates. It is missing a centrum making it ring-like. It has ne r tw cncavities at the cranial end fr articulatin with the ccipital cndyle(s) f the skull. This allws the skull t rck in a ndding mtin nly. The secnd vertebra, the axis, has a cranial extensin, the dntid prcess which is believed t be the centrum f the atlas. It prjects frward and inserts int the flr f the atlas. There is usually a reductin in zygapphyses and ribs n which the skull and atlas rck. Subsequent cervical vertebrae als have mre mbile articulatins giving rise t a highly flexible neck, which reaches its greatest in birds and turtles. The cmbinatin f 1) the atlas-axis cmplex, 2) a mbile articulatin between cervical vertebrae and 3) a large number f cervical vertebrae -give rise t a very flexible neck.
The Sacrum and Synsacrum In many vertebrates the sacral vertebrae have becme fused int a single sacrum. This increases the strength f the cmplex t withstand the thrust f the pelvic girdle and hind limbs during lcmtin. In birds, the demands f flight lead t fusins and flexins f different parts f the clumn. The synsacrum frms (sacrum and adjacent vertebrae fuse with the inminate (ilium, ishium, pelvis) t prduce a stable platfrm while the head and cervical spine are quite flexible t cmpensate. In tetrapds, the clumn supprts the bdy and receives and transmits the frces frm the limbs that generate lcmtin. Since diverse frces are placed n different parts f the clumn, nt surprisingly we see differentiatin f specialized regins. The trunk regin subsequently differentiates int the thraclumbar regin, which ultimately becmes the thrax and lumbar regins. The thrax has ribs and the lumbar regin des nt. In tetrapds, the clumn supprts the bdy and receives and transmits the frces frm the limbs that generate lcmtin. Since diverse frces are placed n different parts f the clumn, nt surprisingly we see differentiatin f specialized regins. The rigin f a separate, ribless lumbar regin is believed t reflect increasing speed f lcmtin n land. The presacral regin is the regin that experiences the greatest lateral flexin during terrestrial lcmtin in quadrapeds and it is thught that the ribs interfered with rapid mvement and thus they were subsequently lst. Thus, in mammals, there are five distinct regins t the vertebral clumn, cervical, thracic, lumbar, sacral and caudal. Fluid Envirnment Fr aquatic rganisms, the endskeletn des nt play much f a rle in supprt. Mst animals are clse t being neutrally buyant and their weight is supprted by the water all arund them. Terrestrial Envirnment The transitin frm water t land was accmpanied by the mechanical demands placed n the axial skeletn.
Fr terrestrial animals, gravity becmes a prblem. They must either rest fully n the grund r be supprted by legs. Bridging Frces acting n pillars: a) When a material bends under a lad, cmpressive frces develp alng the cncave side and tensile frces alng the cnvex side. b) When the supprtive clumn is laded symmetrically the nly type f frce experienced is cmpressive frce. c) Asymmetrical lading f the same mass causes the clumn t bend. The clumn experiences cmpressive frces and tensile frces that are greater near the surface and diminish twards the centre f the clumn. Frces acting f the spinal clumn: Since the weight f the viscera hangs frm the vertebral clumn between the tw pair f legs, the lad will tend t cmpress the bnes n ne side f the legs and stretch the bnes n the ther side (put them under tensin). These cmpressive and tensile frces can be ffset if equal, and ppsite frces are applied frm the ther directin. This is a frm f cantilevering. In the case f tetrapds, the tail acts as a cantilever at ne end and the head at the ther. Nte, the vertebral clumn itself will have frces acting t cause it t bend, placing the clumn under cmpressin n ne side. The ligaments that jin the vertebrae tgether will ppse this tendency by being placed under tensin. In the case illustrated here, the cervical vertebrae are place under cmpressin n the bttm and the ligaments running frm the back f the skull dwn the clumn ppse this frce and are placed under tensin ( as in the case f the vilin bw). The thracic and lumbar vertebrae are als placed under c mpressin n the bttm (nte the directin f the arc) and the ligaments running frm the sternum t the pelvic girdle ppse this frce and are placed under tensin. As lng as the frces are equal and ppsite, the structure is slid and stable.
Design f Vertebrae Angle f the Neural Spines The angle that the neural spine makes with its centrum usually reflects the frces being placed upn it. The tensile ligaments, and the axial muscles attach t the spines and exert frces n them. Muscles will attach frm bth the frnt and the back. The resultant frce placed upn the spine will be the net sum f these frces acting frm bth directins. The spines are strngest when the net sum prduces a frce that acts alng the spine twards the centrum (it is under cmpressin). Als, bnes tend t grw as a result f the frces placed n them. The net result is that the directin f the spine reflects the directin f the resultant frce impsed upn it by all axial muscles inserted n it. (Analgy: try t use a lever t pen smething and think abut hw much easier it is t use a lnger lever.) Vertebral designs incrprate mdificatins t meet mechanical prblems. In many animals the height and directin f the spines within the same vertebral clumn indicate the specialized functins served by different sectins f the vertebral clumn. Ribs and their Derivatives Agnatha have n ribs. The height f the neural spine is prprtinal t the mechanical leverage the muscles must exert t mve r stabilize the spinal clumn. T cunter increasing frces acting n a neural spine, yu culd either use larger and larger muscles, r use a lnger lever (r bth). Ribs develp in ther vertebrates as struts that can either fuse with the vertebrae r articulate with them (ne r tw heads). They prvide sites fr muscle attachment, and frm a prtective case arund the viscera. In sme fishes, there are tw sets f ribs with each vertebral segment in the trunk regin. The drsal ribs separate the epaxial and hypaxial musculature f each segment and grw ut in the hrizntal septum. The ventral ribs frm alng the lining f the celmic cavity. They are serially hmlgus with the hemal arches f the caudal regin. In sme they nly ccur in the drsal psitin while in thers they nly ccur in the ventral psitin.
In tetrapds, the ventral ribs are lst and the drsal ribs persist but take up the psitin f prtecting the bdy cavity. While they play a rle in lcmtin in tetrapds, they becme imprtant fr prducing respiratin. They are bicipital, having tw heads, the tuberculum and the capitulum. In amnites, in the thrax, the ribs cnsist f tw parts, a cstal rib that articulates with the vertebrae, and a sternal rib that ften articulates with the sternum. The latter may remain cartilagenus. This cmbinatin f vertebrae, cstal and sternal ribs and sternum, frms a basket that prtects the thracic viscera. Sternum This structure is absent in fish and first appears in tetrapds. It is derived frm either the ribs r the pectral girdle and becmes assciated with bth. It has evlved independently several times and cnfers stability t weight bearing girdle elements. Its size is a functin f the extent t which the frlimbs are used fr lcmtin. It serves fr flight muscle attachment in birds.