INTRODUCTORY. JOINTS. 13 



the other with the obliteration of the intervening suture. This 

 feature is especially well marked in the cranium of most birds. 



The various kinds of joints or articulations 1 may be sub- 

 divided into imperfect joints and perfect joints. 



In imperfect joints, such as the in tervertebral joints of 

 mammals, the two contiguous surfaces are united by a mass 

 of fibrous tissue which allows only a limited amount of motion. 



In perfect joints the contiguous articular surfaces are 

 covered with cartilage, and between them lies a synovial 

 membrane which secretes a viscid lubricating fluid. 



The amount of motion possible varies according to the 

 nature of the articular surfaces ; these include 



a. ball and socket joints, like the hip and shoulder, in 

 which the end of one bone works in a cup provided by an- 

 other, and movements can take place in a variety of planes. 



b. hinge joints, like the elbow and knee, in which as 

 in ball-and-socket joints one bone works in a cup provided 

 by another, but movements can take place in one plane only. 



THE ENDOSKELETON. 



The endoskeleton is divisible into axial and appendi- 

 cular parts ; and the axial skeleton into 



1. the spinal column, 



(a. the cranium, 



2. the skull {...,,. . 111 + 



[o. the jaws and visceral skeleton, 



3. the ribs and sternum 2 . 



I. THE AXIAL SKELETON. 



1. THE SPINAL COLUMN. 



The spinal column in the simplest cases consists of an 



1 See Huxley's Elementary Physiology, Revised edition, London, 1886, 

 p. 180. 



2 Strictly speaking the jaws, visceral skeleton, ribs and sternum do 

 not form part of the axis, but it is convenient to group them as parts of 

 the axial skeleton. 



