18 THE BONES 



absorption, which ultimately receive bloodvessels, and become the 'vascular 

 canals.' The immediate nutrition of bone is provided for by the proauction 

 of minute ' plasmatic canals' from the vascular ones. When these canals 

 become dilated, so as to offer definite forms, they are termed ' lacunae or 

 'bone-cells,' and to some extent characterise, by their shape and size, tlie 

 osseous tissue of the respective vertebrate classes. In the concentric 

 laminae surrounding the vascular canal, the bone-cells or osteoplasts are 

 arranged concentrically, between the laminae, with the long axis m the 

 direction of the circular line of the plate. Most of the plasmatic tubes con- 

 tinued from the bone-cells pierce the plates at right angles in their course 

 to the vascular canal, with which they communicate ; and they form the 

 essential vehicle of the material for future growth. Extension of parts, 

 however, is not the sole process which takes place in the growth of bone ; 

 to adapt it to its destined offices, changes are wrought in it by the removal 

 of parts previously formed. In marine creatures, the bones usually remain 

 solid ; but in the active land quadrupeds, the shaft of the long bones is 

 hollow, the first-formed osseous substance being absorbed, as new bone is being 

 deposited without. The strength and lightness of the limb-bones are thus 

 increased after the well-known principle of the hollow column. The bones 

 of birds present this quality in the highest degree, particularly those of 

 powerful flight. In these the medullary cavity of beasts is transformed into a 

 capacious cavity containing rarified air instead of marrow. In the mam- 

 malian class, the air-cells of bone are confined to the head, and are filled 

 from the cavities of the nose or ear, not from the lungs, as in birds. Such 

 cells are called ' frontal sinuses,' ' antrum,' ' sphenoidal,' and ' ethmoidal.' 

 The frontal sinuses extend backward over the top of the skull in the rumi- 

 nant and some other quadrupeds, and penetrate the cores of the boms in 

 oxen, sheep, and certain antelopes. The most remarkable development of 

 cranial air-cells is presented by the elephant, the intellectual physiognomy 

 of this large quadruped being caused, as in the owl, not by the actual 

 capacity of the brain-case, but by the vast extent of the pneumatic cellular 

 structure between the outer and inner plates of the skull-wall. All these 

 varied changes in the osseous tissue, from mere cancelli to large medullary 

 or pneumatic cavities, are the result of secondary changes by absorption, 

 and not of the primitive constitution of bones, which were at first solid.") 



CHAPTEE II. 



THE BONES OP MAMMALIA IN PAETICULAE. 

 Aeticle I. — Vbrtebkal Column. 

 The vertebral column, or spine, is a solid and flexible stalk situated in the 

 middle and upper part of the trunk, of which it forms the essential portion. 

 It protects the spinal cord and sustains the thorax, as well as the principal 

 "'•fwl V. ^^ circulation, respiration, and digestion. Articulated anteriorly 

 with the head, and terminating in a point at its posterior extremity this 

 piece IS formed by a somewhat considerable assemblage of short single 

 tuberous bones, to which has been given the name of vertehree ' These' 



