54 



their ramifications are so thickly grouped together, that in very thin 

 and uncut fragments of bone, they form so close a net-work over the 

 bone-cells as almost to obscure them by their opacity. The surface 

 of such bones will present a minutely-dotted appearance, which is 

 caused by the numerous canaliculi which open upon it. If the sec- 

 tion be very thin, it may be seen that the dots are really perforations 

 of the bone. The cells attain their greatest length in the compact 

 bones, such as those of the extremities, but in the flat bones they are, 

 generally speaking, much shorter and broader; and if the bone be a 

 recent one the contents of the cells may be distinctly seen. It will 

 often happen that in a transverse section there may be noticed cells 

 which appear at first sight like those of the Mammalia, they being of 

 a rounded figure, and having their canaliculi arranged like those of 

 the latter class. When they are more carefully examined, it will be 

 found that they are nothing more thau long cells cut across their short 

 diameters. 



In fishes we have a greater variation in the minute structure of the 

 skeleton than in either of the three classes already noticed ; and there 

 are certain remarkable peculiarities in the bones of fishes, which are 

 so characteristic, that a bone of one of these creatures can never be 

 confounded with that of any animal of a higher class, when once the 

 true structure has been satisfactorily understood. We lose now those 

 characters which were evident in mammals and birds, and often, to a 

 certain degree, in reptiles, viz., the concentric laminated arrangement 

 of the bony matter, with a corresponding concentric arrangement of 

 bone-cells around the Haversian canals ; and we shall find that the 

 true Haversian canals may be even absent in some fishes, their place 

 being supplied by numerous layers of bone-cells. All the varieties 

 of structure in the bones of fishes which I have yet made out may be 

 referred to one of the following heads : by far the greater number of 

 bones exhibit nothing more than a series of ramifying tubes, like 

 those of teeth ; others exhibit Haversian canals, with numerous fine 

 tubes or canaliculi, like ivory tubes, connected with them ; others 

 consist of Haversian canals with fine tubes and bone-cells ; whilst a 

 rare form, found only as yet in the sword of the Sword-fish {Istiophorus), 

 exhibits Haversian canals and a concentric laminated arrangement of 

 the bone, but no bone-cells. The Haversian canals, when they are 

 present, are of large size and very numerous, and then the bone-cells 

 are, generally speaking, either absent or but few in number, their 

 place being occupied by tubes or canaliculi, which are often of large 

 size. The bone-cells are remarkable for their quadrate figure, and 



