20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



units in some and occur very infrequently in other individuals. In 

 the flat bones of the cranium and in the irregular bones like those 

 of the pelvis a few are found where it is difficult, if not impossible, 

 to understand how they could perform any mechanical function of 

 any value. 



In the human frontal, parietal and occipital bones, a few Haver- 

 sian systems may be found at right angles to each other ; in tlie 

 squamous portion of the temporal bone they extend vertically ; in 

 the hard palate, antero-posteriorly ; in the superior and inferior 

 maxillae, horizontally ; in the spine of the scapula, horizontally ; in 

 the crest of the ilium horizontally ; and in the ribs horizontally. In 

 these different situations the mechanical functions of the Haversian 

 systems do not seem to answer any requirement which is common 

 to them all. Furthermore, they are not invariably found in these 

 situations and even may not be present at all. Their presence or 

 absence or situation within the bone is more satisfactorily explained 

 if we assume that developmental advances in the circulation deter- 

 mine the types of bone. 



CONCLUSIONS 



A study of the circulations as they were observed in the foregoing 

 slides leads one to the following conclusions : 



1. That a cross section of bone gives no idea of the plan of 

 circulation. 



2. That there are two types of circulation, the branching and the 

 plexiform, and these are seen only in tangential sections. 



3. That the plan of circulation in bone derived from connective 

 tissue is branching; derived from cartilage, plexiform. 



4. That the circulation in a first type bone is branching, in the 

 second and third, plexiform. 



5. T^hat the plexiform is a more advanced type of circulation than 

 the branching and represents a later plan of vascular distribution 

 and a more advanced degree of bone differentiation. 



6. That bone is a very vascular organ. 



7. That blood vessels are composed of extremely thin and finely 

 striated connective tissue walls, without smooth muscle, occupying 

 canals in the bone substance. 



8. That vascular expansions occur in bone circulations and, by 

 their positions, suggest equalizing blood pressures and uniformity 

 in the circulation. 



9. That developmental advances in the circulation determine types 

 of bone. 



