l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



DEVELOPMENTAL ADVANCES IN THE CIRCULATION 

 DETERMINE THE STRUCTURAL TYPES OF BONE 



It is generally supposed that bone is preeminently mechanical in 

 function on account of its position in the body and its rigid character. 

 It forms the skeleton of the animal body, supports its weight, gives 

 attachment to muscles by means of which locomotion is possible, 

 serves as a framework upon which the viscera are hung, affords pro- 

 tection and gives efficiency to the laboratory of chemical activities 

 constantly in operation during life. It is chiefly for these reasons 

 that we attribute a mechanical function to bone. This thought is 

 further strengthened by the microscopic structure so familiar to us 

 either from personal observation or from text books of histology, 

 in both of which bone is represented, usually, as composed of Haver- 

 sian systems. 



There are however, as shown before, three structural types of 

 bone, the first, second, and third. 



The first type is composed of bone substance or lamellae, the 

 second of laminae which are produced by a vascular separation of 

 lamellae into parallel divisions, and the third, of Haversian systems 

 which are produced by arranging lamellae around small central vas- 

 cular canals called Haversian canals. The Haversian system, there- 

 fore, is the most complex and highly organized bone unit. The types 

 of structure follow the advancing changes in the circulation, since 

 the second type bone is not recognizable until the first has been separ- 

 ated into laminae by parallel vascular canals. There are two general 

 types of circulation, the branching and the plexiform, each one giving 

 an individual character to the type of bone it produces. Bone de- 

 rived from connective tissue membranes retains the branching circu- 

 lation of those membranes, while bone derived from cartilage has 

 the plexiform circulation. 



In going from a branching to a plexiform type of circulation, the 

 type of bone advances from the first to the second or third. The 

 blood supply in a plexiform circulation is greater in volume than it 

 is in a branching circulation, for the reason that there are more blood 

 vessels in a given area in the former than in the latter. This in- 

 creased blood supply adds an increased physiological value to the 

 bone units of structure and is the foundation of advancement in 

 tissue values : so that blood supply is determined by the extent, plan 

 of distribution, or type of circulation and the type of tissue, phys- 

 iologically considered, by the blood supplv, or in this particular 

 instance, the type of bone is determined by the type of circulation. 



