30 LECTURE II. 



In most osseous fishes the nucleus of the cartiUxge-cell quite dis- 

 appears in the process of ossification, and only the tubular prolon- 

 gations of the nuclear matter leave permanent traces, as plasmatic 

 tubes, which traverse the osseous lamellae in the intervals of the vas- 

 cular canal, and freely open into the latter, which are unusually 

 numerous. In the bones of Reptiles there is more diversity in the 

 number of the Haversian canals, which is less in Oj)hidians and 

 Saurians than in the fish-like Batrachian. The radiated cells are 

 always present, but are less regular in form and rather larger than in 

 Mammals ; they are rounder in Birds, with less conspicuous radiating 

 plasmatic tubes ; they are usually more elliptic and compressed in 

 Mammals, in the osseous tissue of which class the more appreciable 

 difference in microscopic structure obtains in the relative size of the 

 Haversian canals to the plasmatic, calcigerous, radiated cells. 



These cells vary little in diameter in different Mammalia ; but the 

 Haversian canals which average ^^q*^ °^ ^^ inch in diameter in the 

 mouse are j^o^l^ of an inch in diameter in the ox, and ^^o*^^ ^^ ^^ 

 inch in the human subject. 



The Haversian canals are fewer in the dense osseous tissue of 

 Birds than in that of Mammals : in the bones of Chelonia and 

 Batrachia they are more numerous, larger, and more reticularly dis- 

 posed ; the radiated cells are also larger. 



In my treatise on the teeth (v. pp. xvi — xxii.), I have shown that 

 the osseous tissue corresponds in microscopic structure with that of 

 the dentine in many fishes. In no class is the structure of the teeth 

 more varied, and in none do we find such extreme modifications in 

 that of the bones. 



Throughout a great part of the skeleton of the pike the osseous, 

 like the dental, tissue is characterised by " a reticulo-medullary 

 tubular structure : " the meshes or interspaces being traversed by the 

 rich series of plasmatic tubes communicating with the vascular or 

 Haversian canals ; and there are few central dilatations radiating 

 plasmatic tubes, in other words, few purkingian cells. 



In this section of the lower jaw of a Muraena (Prep. 2560 a.) we per- 

 ceive, on the contrary, an abundance of radiated cells, but no Haversian 

 canals. The cells, divided lengthwise, present a long, thin ellipse, with 

 the ends prolonged into plasmatic tubes, larger than those which radiate 

 from the sides ; and, as the terminal prolongations communicate Avith 

 each other, a series of cells may often be traced resembling a monili- 

 form or alternately dilated and contracted canal. In a section of a 

 jaw of the common eel, large, irregular vascular canals are seen, 

 combined with radiated cells like those in the Murasna. In a section 

 taken from the second thin longitudinal crest of the cranium of an 



