i66 



OSSEOUS SYSTEM. 



[sect. 88. 



disposed as to constitute a pretty regular network. If the spaces 

 are larger, the substance is called substantia cellularis ; if they are 

 smaller, substantia reticularis. The latter, in some places, where 

 its spaces become narrower, and the osseous trabecule thicker, 

 approximates in structure to the compact osseous substance, 

 without, however, being really converted into it, and, in other 

 places, passes, without any sharp line of demarcation, into compact 

 tissue. This, however, does not prove that both are identical, 

 but, as shown by their development, simply depends upon the 

 circumstance, that the spongy substance frequently arises from 

 the opening up of the compact by partial absorption. The above- 

 mentioned two substances are very variously concerned m the 

 formation of the bones and different parts of bones. The com- 

 pact substance is found alone and without vascular canals, only 

 in a few places, as in the lamina papyracea of the ethmoid bone, 

 some parts of the lacrymal and palate bones, etc. j but more fre- 

 quently with vascular canals, as in the thinnest parts of the 

 scapula, ilium, acetabulum, and flat cranial bones (ala magna, 

 parva, processus orbitalis ossis frontis) in many individuals. Spongy 

 tissue, with a thin compact cortex without vascular canals, exists 

 in the ossicles of the ear, on all the surfaces of bones which are 

 covered with cartilage, and perhaps, also, in small spongy bones. 

 In all other, and, therefore, the most numerous cases, the two 

 substances occur together, but in a such a manner, that sometimes 

 the spongy substance preponderates (spongy bones and parts of 

 bones), as in the vertebra, carpus and tarsus, sometimes the 



Fig. 75. 



Segment of a transverse section, from the diaphysis or shaft of the femur of an individual 

 eiehteen years old ; magnified 25 times, a. Haversian canals ; 6. opening of the same 

 inwards ; c. outwards; d. osseous substance with lacunse. No transverse sections of vas- 

 cular canals or principal lamellas are seen. 



compact, as in the diaphyses of the long bones; or both are 

 present in about the same proportion, as in the fiat bones. 



