BONE. 



6l 



lacuna to another, connecting the different Haversian systems, and opening 

 into the Haversian canals. 



THE RELATION OF BONE TO CARTILAGE. 



Some bones develop quite independently of cartilage. These include, 

 besides the teeth, the so-called membrane banes [intramembranous, connec- 

 tive tissue or secondary bones]. In the midst of the embryonic connective 

 tissue, spicules of bone 

 are formed in the man- 

 ner already described, 

 and they unite to form 

 a bone. The membrane 

 bones are the bones of 

 the face, and the flat 

 bones of the skull; the 

 interparietal or upper 

 part of the occipital, the 

 squamous and tympanic 

 parts of the temporal, 

 the medial pterygoid 

 plate of the sphenoid, 

 the parietal, frontal, 

 nasal, lachrymal, zygo- 

 matic (malar), and pal- 

 ate bones, together with 

 the vomer, maxilla, and 

 almost the entire man- 

 dible. The remaining 

 bones, being preformed 

 in cartilage, are grouped 

 as cartilage bones [pri- 

 mary bones]. They de- 

 velop like membrane 

 bones except that the matrix is in part deposited in contact with cartilage 

 in the following manner. 



Fig. 65 shows a longitudinal section of a developing phalanx. On 

 either side of the shaft a strip of bone is seen, formed from undifferentiated 

 cells of mesenchymal origin, situated in the perichondrium. It is called 

 perichondral or periosteal bone, and arises like membrane bone. As a 

 whole, it forms a band encircling the shaft of cartilage. Within it, the 

 cartilage cells have enlarged, and divided so that several cells may be in 



Endochon- 

 dral bone. 



Perichon 

 dral bone. 



Periosteum. 



FIG. 66. FROM A DORSO-PALMAR LONGITUDINAL SECTION OF A 

 MIDDLE-FINGER PHALANX OF A HUMAN FETUS FOUR MONTHS 

 OLD. X 60. 



