[7] THE OSTEOLOGY OF AMIA CALVA. 753 



eye, reaches the preorbital, and terminates laterally near the anterior 

 nasal aperture. 



From the mucus canal leading to the orbital arch another canal takes 

 origin, beginning in the frontal, passing through the entire length of 

 the squamosal, to enter the extrascapula and suprascapula. After 

 passing through the suprascapula it becomes the mucus canal of the 

 lateral line, passing on to terminate at the tail. Both of these canals, 

 just referred to, are united by a transverse anastomosis, which is im- 

 bedded in the substance of the extrascapula. During its course through 

 the squamosal a branch directed laterally arises from this canal. This 

 branch enters the preoperculum, passing through the entire length of 

 this bone to enter the mandible beyond, and eventually join the fellow 

 of the opposite side, which it meets at the symphysis. All these mucus 

 canals send off numerous ramifications of smaller canals, arranged in 

 several longitudinal rows, which terminate on the surface of the head in 

 minute openings. 



Taking into consideration their superficial location, the peculiar sculp- 

 turing of their surface, and the possession of mucus canals, the bones 

 we have Just described are unquestionably characterized as ossifications 

 of the skin — as dermal bones. In making any attempt to remove these 

 dermo-bones one recognizes the fact that their relations to the chondro- 

 cranium are very different. 



The ethmoid, the nasals, and the preorbitals ^^ do not come in contact 

 at all with the same, but are separated from it throughout their entire 

 extent by soft parts. 



On a microscopical examination of cross-sections made from one of 

 these bones (take for example the extrascapula) one can distinguish 

 a superficial layer from a deep one. The latter consists of osseous 

 lamellae, which are piled up parallel with the bony plane, and which are 

 interrupted by others, arranged concentrically around the Haversian 

 canals. 



This deeper bony layer gives passage to quite a number of capacious 

 Haversian canals and is supplied pretty generously with bone corpuscles. 

 The superficial layer of these dermal bones is characterized, when com- 

 pared with the one just described, by a much denser tissue, by a small 

 number of Haversian canals, by an almost entire absence of bone cor- 

 puscles, and, what is most important, by the existence of numerous and 

 very minute dentine tubelets {Dentinrdhrchen) which penetrate it from 

 the surface of the bone. Yet I wish to explicitly state that one cannot 

 make out the exact boundary between these two layers with any cer- 

 tainty. 



The frontals, parietals, and squamosals are in more intimate relation 

 with the skull. In part, these are quite closely connected with the car- 

 tilaginous cranium, and are separated from it simply by a layer of thin 

 connective tissue. Histologically they remind one very much of the 



'^ This applies also to the extraaoapnlar, the suprascapular, and the supraclaviculas. 

 H. Mis. 67 48 



