176 OSSEOUS SYSTEM. [SECT. 92, 93. 



secondary cell-membranes of the cartilage-cells, which membranes 

 have become ossified and coalesced with the intermediate substance. 

 In the cement of the tooth of the horse, the secondary capsules, 

 as I find, can also be isolated with the bone-cells by maceration in 

 hydrochloric acid. 



Tomes and Be Morgan describe in the circumferential laminse of bones 

 a peculiar kind of canaliculi, in form of elongated tubes, which pass in 

 bundles or singly, more or less obliquely from the surface towards the interior 

 of the bone. When long, they are sometimes bent once or twice at a sharp 

 angle. They have parietes, and are connected laterally with the canaliculi. 



§ 92. Periosteum. — This is a slightly glistening, yellowish- 

 white, vascular, extensile membrane, which covers the greater part 

 of the surface of the bones, and from the numerous vessels which 

 it sends into their interior, is of the greatest importance for their 

 nutrition. 



The periosteum, which is sometimes thicker, sometimes thinner, 

 presents almost everywhere, excepting only where muscles arise 

 directly from it, two layers, which, although firmly united to each 

 other, can be more or less perfectly distinguished by their struc- 

 ture. The outer layer is formed principally of connective tissue, 

 and occasionally a few fat cells, and is the principal seat of the 

 vessels and nerves belonging to the membrane ; whilst in the inner 

 layer, elastic fibres, generally of the finer kinds, form continuous 

 and often very dense net-works — in fact, real elastic membranes 

 superimposed in several layers — the connective tissue forming the 

 less important element. Nerves and vessels also occur in this 

 layer, which, however, only pass through it, being destined for 

 the bone itself. 



§ 93. Marrow.— The marrow, which is found in all the larger 

 cavities of the bones, with the exception of most of the Haversian 

 canals, and the smaller nervous canals of the flat cranial bones, 

 appears in two forms, namely, yellow 

 and red. The former is found as a 

 softish mass, especially in the long 

 bones, and consists of 96-0 fat, ro 

 areolar tissue and vessels, and 3-0 of 

 fluid with extractive matters; whilst 

 the latter occurs in many apophyses, 



in the flat and short bones, but above Two fat cells, from the marrow of 



the human thigh-bone ; masnnhed 



all in the bodies of the vertebrae, the 350 times. «. Nucleus ;£,ceii-mem- 



brane ; c contained oil-globule. 



