328 THE TISSUES. 



and hence this form of bone-substance seems rather to present an 

 interlacement of lamellae, rods, and fibres, very irregularly arranged. 

 (Fig. 216.) If the connecting rods are of considerable size, they 

 contain vascular canals; otherwise, merely laminae, lacunae, and 

 pores. The lacunae are disposed in every possible direction ; but 

 mostly with their long axes parallel to that of the fibres and rods, 

 and with their flat surfaces directed towards the cancelli, into which 

 the most superficial lacunae freely open. The vessels of different 

 cancelli freely anastomose with each other. 



The cancelli contain -first, the vessels already mentioned; se- 

 condly, a prolongation of the periosteum or endosteum supporting 

 these vessels ; and, thirdly, more or less fat-cells with red contents 

 (marrow). Nerves also, fourthly, are distributed to the marrow, 

 especially in case of the bodies of the vertebrae. 



2. Compact Bone-structure. The thinnest layers of compact tissue 

 consist merely of parallel superimposed lamellae, between and in 

 the substance of which lacunae and pores (but no vessels) exist; 

 e. g. some portions of the lachrymal and palate-bones. Indeed, the 

 wall of a cancellus, as before described, is essentially a very thin 

 layer of compact bone-structure. 



When the compact structure, however, attains to a thickness of 

 g 1 ^ of an inch, and more, a different arrangement of the lamellaB is 

 found. And in the long bones the compact substance consists of 

 two systems of lamellae (the general and the special), and the inter- 

 stitial osseous tissue, or that between the Haversian rods. 



1. The general (fundamental) lamellae are parallel to the^xternal 

 and the internal surface of the bone. These alone exist where the 

 compact substance is very thin. They, however, rarely entirely 

 surround the long bones, and are absent in the fast-growing bones 

 of young animals. 1 (Tomes and De Morgan.) They are in immediate 

 connection at many points with those next to be described, and are 

 seen in Fig. 217. The lacunae are placed with their surfaces parallel 

 with those of the lamellae ; their pores opening in part on the ex- 

 ternal and internal surfaces of the bone, and in part, communicating 

 with each other, though they probably terminate in blind extremi- 

 ties at points covered by the articular cartilages. The thickness of 



' In these the circumferential laminse are replaced by a series of undulating 

 laminae, which, subsequently extending outwards, arch over and inclose the nearest 

 vessels of the periosteum ; and in the spaces thus formed, Haversian rods are de- 

 veloped (p. 357). 



