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PinkMonkey Online Study Guide-Biology

21.1 Joints - Articulations between Bones

A joint is that part of the endoskeleton where two or more bones meet. Joints between bones are classified into three main types on the basis of both structure and function, though these are, of course, related.

(A) Fibrous or Immovable Joints (Synarthrosis)

This includes all immovable joints in which the bones are united by a thin or dense layer of fibrous tissue. The cracks between the bones are called sutures (Figure 21.1). These joints are places of growth and remain open till the growth period is over, after which they tend to ossify. (Most sutures in marsupials remain open for life).

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Figure 21.1 Sutures of Skull

As per the configuration of the sutures, the following sub-types are present (Figure 21.2).

(i) Butt joint. Sutures are straight or have squared off edges. Butt joints can withstand compression, but not shear or bending. (e.g., two nasal bones).

(ii) Lap joint. When the squared edges of bones overlap, it forms the lap joint. Lap joints are stronger, and unusual in animals.

(iii) Scarf joint. If the overlapping edges taper so that the two bones remain in line, it is called a scarf joint. This kind of a joint improves strength. (e.g. various skull bones).

Figure 21.2 Subtypes of fibrous joints

(iv) Peg and socket joint. Small projections are formed in one bone which fits into another bone. (e.g., thecodont teeth to the jaw bones, epiphysis of long bones to their shafts).

(v) Serrate joint. The bones have irregular suture, so the bones interlock repeatedly through the union. (e.g., roofing bones of the skull).

(B) Cartilaginous or Slightly Movable Joints (Amphiarthriosis)

In this joint, the adjoining bony surfaces are connected by cartilage. It is a tough joint and allows some motion in response to compression, tension or twisting.

(i) The union between the bones of such a joint is called a symphysis instead of a suture (Figure 21.3 A) (e.g., pelvic symphysis between pubis and pelvis, madibular symphysis, and vertebral centra, which provide for motions of the spine). The pubic symphysis is more flexible in women under the hormonal environment during pregnancy; this is needed for childbirth, to enable the birth canal to enlarge.

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Table of Contents

21.0 Introduction
21.1 Joints - Articulations between bones
21.2 Muscle movements

Chapter 22


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