Page:The American Cyclopædia (1879) Volume III.djvu/65

 BONE 59 verse branches ; they thus form a network of tubes for the minute vessels which they con- vey and protect. According to Todd and Bowman, the arteries and veins usually occupy distinct Haversian canals, a single vessel being distributed to each. The canals conveying the veins are said to be the larger, and to present at irregular intervals, where two or more branches meet, pouch-like sinuses which serve as reservoirs to delay the escape of the blood ; in some of the irregular bones, as in those of the skull, the venous canals are extremely tor- tuous, running chiefly in the cancellated struc- ture, there called diplot. The Haversian canals vary in diameter from j-J^ to j-j 1 ^ of an inch, the average being about ^-j^, and their ordi- nary distance from each other about -j-J-y of an inch. This whole apparatus of canals is only an involution of the surface of the bone, that the vessels may come into a more free contact with it ; as they communicate internally with the medullary cavity, externally with the pe- riosteal surface, and also with the cancellar medullary cells, the network of nutrient ves- sels is very complete. But, as if this arrange- ment were not enough to secure the nourish- ment of such a hard tissue as bone, and so far removed from immediate contact with blood vessels, there is a still more curious and deli- cate apparatus of microscopic cavities. Around the Haversian canals will be noticed the ap- pearance of delicate lamella of bone, more or less concentric; these, with the lacunae men- tioned below, are the most essential constitu- ents of true and fully developed bone, the med- ullary cells and Haversian canals being merely definite spaces existing between the lamellaa. It is principally by the successive development of new lamellse that bones increase in diameter, being usually deposited in the direction of the axis. A transverse section, therefore, would present under the microscope the following arrangement of lamellas, as given by Hassall : 1, several layers passing entirely round the bone; 2, others encircling each Haversian canal ; and 3, irregular and incomplete lamellae occupying the angular spaces between those concentrically arranged. The lamellse of the Haversian canals, however, are not exactly concentric, as commonly described, but incom- plete and running into one another at various points, a necessary consequence of the irregu- lar distribution of the lacunsa. The Haversian systems generally run in the direction in which the tissue requires the greatest strength. With the previously mentioned arrangement of the cancellated structure, the Haversian canals more fully display the wonderful adaptation of means to ends, combining mechanical advan- tages with the best provisions for the nutrition of the tissue. The number of lamellae passing entirely round the bone is generally less than 12, and those encircling each Haversian canal vary from 2 or 3 to more than 12, the smallest canals having the fewest lamellte. The lamel- la;, according to the best observers, appear to consist of a delicate network of fibres in sets, the fibres of each set running parallel, but crossing the others obliquely ; some have sup- posed that they are produced by the union of a number of diamond-shaped cells, and not by the crossing of fibres ; the first opinion is prob- ably the true one. Distributed through the cancellated and compact portions of bone oc- cur numerous black specks in the lines of the lamellse ; these are the lacunae, or bone corpus- cles, the most peculiar and characteristic mi- croscopic form to be found in bony tissue. They differ somewhat in form in different ani- mals, but are always more or less flatten- ed, elongated, ovoid bodies, with numerous branches and radiating filaments passing out from them and communicating with those in the adjacent lamellffi. In the dried bone the lacunae are empty, owing to the decomposition 3'f [  ' >#' "Hi; r ,v. FIG. 1. Transverse section of bone in the neighborhood of two Haversian canals, a a; &, lacunsu. -?1 ! IP ?.:*>! *J ;f*. : lf*-l Fia. 2. Longitudinal section of bone with Hayersian canals, a a, and lacun*. b (less magnified than the preceding). FIG. 8. Lacuna;, c, and canaliculi, t/, very highly magnified. and shrinking of the soft parts, and the branched lines running out from them appear as minute canals or canaliculi; but in the fresh condi- tion they are both undoubtedly filled with a