II.] 



THE SPIRAL SKELETON. 



other than those observed in the human body. Thus some 

 spiny bones of Siluroid fishes have a perforation at their 

 base, through which passes a bony ring attached to a plate 

 below a shackle-joint. This structure, however, belongs to 

 the external skeleton. 



5. The parts of the endoskeleton may obviously be grouped 

 into two divisions : 



(a) The skeleton of the head and trunk, which is called 

 the AXIAL 1 skeleton. 



() The skeleton of the limbs, which 

 is called the APPENDICULAR skeleton, 

 the limbs being regarded as appendages 

 of the axial skeleton. 



First with regard to the axial skele- 

 ton. The skeleton of the head (i.e. the 

 skull) is supported on the very summit 

 of what is familiarly known as the back- 

 bone, while from each side of one region 

 of the latter the ribs reach forwards to 

 or towards the breastbone. 



The skull is of so complex a struc- 

 ture as to require separate consideration. 

 The skeleton of the trunk only (that 

 is to say, the backbone with the ribs 

 and breastbone) will afford material 

 enough for this lesson. 



6. The backbone, or, as it is often 

 called, the spine, 2 consists of a number 

 of small bones placed one on the top of 

 the other like a pile of coins. 



Each of these small bones is termed a 

 vertebra, 3 and (with certain few excep- 

 tions, to be noticed later) consists of a 

 sort of irregular ring of bone, thickest in 

 front, from which certain bony promi- 

 nences stand out in various directions. 



By the superposition of their rings, 



they together form a long vertical canal 



(called the vertebral canal), which is 



' destined to contain and protect the spinal marrow (or spinal 



cord). This series of vertebrae thus constitute the smaller, 



' From being the skeleton of what is, as it were, the axis of the body. 

 2 The word "spine" is also frequently employed to denote any slender and 

 more or less pointed prolongation of a bone . 



/ From vttre, to turn, thcujgh the mobility of most vertebrae is but slight. 



_. 3. Axial skeleton 

 of the trunk with the 

 ribs of the right side 

 removed to show the 

 vertebrae more dis- 

 tinctly. 



