SKELETAL SYSTEM: AXIAL

I.	Vertebrae

	A.	Arose as adjunct to notochord, later replaced it.

	B.	Notochordal tissue retained in some primitive tetrapods, contributes to intervertebral 
                 discs in mammals.

	C.	Comprised of several components, exhibiting various degrees of fusion, often into 
                 single unit.
		1.	Dorsal arches: around neural tube (spinal cord).
			a)	Neural arch
			b)	Interneural (intercalary) arch
		2.	Ventral arches: around blood vessels.
			a)	Hemal arch
			b)	Interhemal arch

		3.	Centrum - shapes of articular surfaces affect mechanical properties of vertebral 
                          column.
			a)	Acoelous = flat ends, suited for compressive forces.
			b)	Amphicoelous = concave surfaces; allows limited motion in most directions.
			c)	Procoelous = concave anteriorly, convex posteriorly.
			d)	Opisthocoelous = concave posteriorly, convex anteriorly.

				Procoelous and opisthocoelous types permit large range of motion without 
                                stretching the spinal cord.

			e)	Heterocoelous = saddle-shaped articular surfaces at both ends.
				Allows extensive lateral and vertical flexion, but little or no torsion (twisting). 
 				eg. turtles that retract necks, cervical vertebrae of birds.

		4.	Intervertebral disc = any pad of tissue between articular surfaces of centra.
			a)	Normally  consists of fibrocartilage containing gellike core - mammals.
			b)	Other vertebrates posses pad called intervertebral cartilage.

		5.	Intervertebral ligament - joins rims of adjacent centra, controlling stiffness of 
                        column.

		6.	Trend toward reduction in # of elements to increase stability & strength; reduces 
                         flexibility.

		7.	Trend in tetrapods toward regional differentiation to increase flexibility and allow 
                          specialization of function;  most evident in mammals.

		8.	Sacral vertebrae brace pelvic girdle; fused in amniotes to form sacrum:
			a.	Reptiles and birds: 2.
			b.	Mammals: usually 3, maybe 2 (5 in humans).

II.	Ribs

	A.	Struts that articulate with or fuse with vertebrae.

	B.	Functions:
		1.	Sites for muscle attachment for locomotion.
		2.	Protect viscera.
		3.	Aid respiration.

	C.	May be two sets of ribs per vertebral segment, dorsal & ventral sets. eg. many 
                 fishes.

	D.	Primitively bicipital, w/ dorsal and ventral articulations.

	E.	Types:

		1.	True ribs: meet ventrally with sternum, two segments:
			a)	Vertebral rib = proximal segment.
			b)	Sternal rib = distal segment, usually cartilaginous, meets sternum.

		2.	False ribs: articulate with each other, but not sternum.

		3.	Floating ribs: articulate with nothing ventrally.

III.	Sternum

	A.	Offers attachment for chest muscles.

	B.	Secures ventral (distal) ends of ribs to complete rib cage.

	C.	May consist of single bony plate or several serial elements (mammals):
		1.	Sternebrae = ossified elements in series.
		2.	Manubrium = 1st element, modified.
		3.	Xiphisternum = last element, modified.

	D.	Absent in turtles, snakes, many limbless lizards.

	E.	Present in some modern amphibians, other reptiles, birds, mammals.

		Absence of sternum in common ancestors of these groups means that the sternum 
                 arose independently several times within the same field of midventral connective 
                 tissue.