LACUNAE AND CANALICULI. 145 



bioplasm only is instrumental in the formation of 

 every one of these tissues, and without this the pro- 

 duction of tissue is impossible. 



20 4. Lacunae and canaliculi. Masses of bioplasm 

 are contained in the little spaces in the bone (lacunae), 

 which are present in every kind of bone tissue, the 

 distance from each lacuna to the neighbouring lacuna 

 being seldom more than y-gVo inch. Moreover, each 

 lacuna is connected with its neighbours by numerous 

 minute channels (canaliculi), along which fluid 

 readily passes from one mass of bioplasm to another. 

 Thus every part of the hard bone tissue is irrigated 

 with fluid, and the little channels are usually not 

 separated from one another by a distance greater than 

 the five-thousandth of an inch, so that no particle of 

 osseous tissue is removed further than half this dis- 

 tance from the fluid which flows in the canaliculi. 

 In the living state, these canaliculi always contain 

 fluid, and this fluid is always in motion, flowing to 

 and from the nearest masses of bioplasm ; but if bone 

 be dried, the bioplasm in the lacunae shrinks, the fluid 

 in the canaliculi disappears, and air rushes in to 

 occupy the spaces and canals thus formed. 



205. Lamellae. The bone tissue is so formed as to 

 constitute a series of superposed thin plates called 

 lamellae. These are concentrically arranged round 

 the vessel in the Haversian canal, and in section they 

 appear as concentric lines one within the other. 

 There are lamellae immediately beneath the periosteum 

 and medullary membrane, which extend uninter- 

 ruptedly for a great length, or entirely round the 

 bone. These are called respectively the periosteal and 

 medullary lamellce. 



206. Perforating fibres. Dr. Sharpey has de- 

 scribed, under the name of perforating fibres, some 

 peculiar processes of osseous tissue which appear to 

 perforate the laminae of bone and, as it were, pin 

 them together. The mode of formation of these fibres 





