116 MICROSCOPIC STRUCTURE OF BONE. 



fewer radiating tubes proceed from the bone cells ; in the os- 

 trich the bone cells are from l-1300th to l-2200th of an inch 

 in their long diameter, and from l-5425th to 1 -9600th in their 

 shortest. In reptiles the Haversian canals are few in number, 

 but large in size, and in the same section we observe the 

 canals and the bone cells arranged both vertically and longi- 

 tudinally. The bone cells in the turtle measure 1-3 75th of an 

 inch in length ; in the amphibia, as the siren, they measure 

 1 -290th of an inch in length. Fishes present considerable 

 variety in the intimate structure of the osseous tissue ; their 

 bone cells have a singular quadrate form ; the ramifying tubes 

 are few in number, and of considerable size, and anastomose 

 freely with the tubes from neighbouring cells, forming thereby 

 a well marked trellis-work in the osseous substance. The 

 specimen before me, a thin section of the scale of an osseous 

 fish, shews this anastomosis most distinctly. The size of the 

 bone cells has been found to bear a remarkable relation to that 

 of the blood corpuscle in the different classes of the ver- 

 tebrata.* 



[ 241. THE HEAD is composed of two parts, the cranium, 

 or skull, and the face. The cranium (fig. 79) is a bony case 

 ,of an oval form, occupying the upper and back part of the 

 head ; it lodges the brain ( 80), and protects it from injury, 

 and in two of its bones is situated the organ of hearing. 

 The walls are formed of the frontal bone (3), which forms the 

 forehead ; the two parietal bones (1) occupy the sides and 

 roof of the skull ; the two temporal (2) form the walls of the 

 temporal region ; and the occipital (4) is situated at the 

 posterior and inferior part. These bones are firmly united 

 to each other by sutures, the character of which varies in dif- 

 ferent parts of the cranium, and their evident intention being 

 to afford the best kind of mechanism for resisting external 

 violence. Thus, a blow upon the vertex tends to separate the 

 parietal bones from each other and from the frontal, and to 

 force their lower borders outwards ; but this accident is admi- 

 rably provided against by the different kinds of sutures which 

 unite the parietal to the frontal, occipital, and temporal bones, 

 thus a serrated suture locks them together above, to the occi- 



* For much valuable information on this subject, consult Mr. John 

 Quekett's papers in the Trans, of the Microscopic Soc. London, vol. ii. part 2. 



