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EXPLANATION OF THE PLATES. 



Plate V. 



Fig. 1. A transverse section of the human clavicle, magnified 95 diameters, which 

 exhibits the Haversian canals, the concentric laminae, and the concen- 

 tric arrangement of bone-cells around them. Some of the Haversian 

 canals are white, others black ; the latter are filled with a deposit of 

 opaque matter, used in the grinding and polishing the section : the 

 outer margin of each of the series of concentric lamina? is white and 

 transparent, which is caused by the canaliculi of the outer row of bone- 

 cells being given off only from one side of the cell, and that side being 

 the one nearest its own Haversian canal. 



Fig. 2. A transverse section of the femur of an Ostrich, magnified 95 diameters. 

 When contrasted with the preceding figure, it will be noticed that 

 the Haversian canals are much smaller and much more numerous, 

 and many of them run in a transverse direction. The bone-cells are 

 very much smaller, and the concentric laminated arrangement only- 

 visible where the canals run at right angles to the section. When the 

 concentric laminae do exist they are not so numerous as in Mammalia ; 

 rarely more than from three to five are found around any one Ha- 

 versian canal. 



Plate VI. 



Fig. 3. A transverse section of the humerus of a Turtle (Chelonia Mydas). It ex- 

 hibits traces of three Haversian canals, with a slight tendency to a 

 concentric arrangement of bone-cells around them : the bone-cells are 

 large and very numerous, but occur for the most part in parallel rows: 

 in these rows the cells are seen in their longest diameters, whilst in the 

 immediate neighbourhood of the Haversian .canals they present their 

 short diameters. 



Fig. 4. A horizontal section of the lower jaw of a Conger Eel, which exhibits a 

 single plane of bone-cells arranged in parallel lines : there are no 

 Haversian canals present ; and when this specimen is contrasted with 

 that of fig. 3, it will be noticed that the canaliculi given off from each 

 of the bone-cells of this fish are very few in number in comparison 

 with that of the reptile. 



Fig. 5. A portion of the cranium of the Menopome, which exhibits numerous 

 bone-cells of a large size : they are arranged in parallel lines, and are 

 much farther apart than those of the Eel or Turtle. No Haversian 

 canal is present, but there is a disposition to a concentric arrangement, 

 as seen on the left-hand side of the figure : at this part the bone-cells 

 are the largest. 



Fig. 6. A portion of the cranium of a Siren (Siren lacertina), which is remarkable 

 for the large size of the bone-cells and of the canaliculi, they being 

 larger in this animal than in any other yet examined ; as in the pre- 

 ceding specimens, no Haversian canals are present. 

 The above four specimens are magnified 95 diameters. > 



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