586 VERTEBRATBD ANIMALS. 



rings, resembling those of an Exogenous Stem. These rings are 

 marked out and divided by circles of little dark spots ; which, 

 when closely examined (2), are seen to be minute flattened 

 cavities excavated in the solid substance of the bone, from the 

 two flattened sides of which pass forth a number of extremely 

 minute tubules, one set extending inwards, or in the direction of 

 the centre of the system of rings, and the other outwards, or in 

 the direction of its circumference; and by the inosculation of the 

 tubules (which are termed canaliculi) of the difierent rings with 

 each other, a continuous communication is established between 

 the central Haversian canal and the outermost pai-t of the bony 

 rod that surrounds it, which doubtless ministers to the nutrition 

 ot the texture. Bloodvessels are traceable into the Haversian 

 canals; but the " canaliculi," being far too minute to carry 

 blood-corpuscles, can only convey a nuti-ient fluid that is sepa- 

 rated from the blood for the special service of the bone. 



404. The minute cavities, or 

 ^^^- ^°°- laeunce (sometimes, but erro- 



neously termed "bone-corpus- 

 cles," as if they were solid 

 bodies), from which the canali- 

 culi proceed, are highly charac- 

 teristic of the true osseous struc- 

 ture; being never deficient in 

 the minutest parts of the bones 



ZdCMnajofOsseoussulistonce— a, central cavity, Qf ^j^g higher Vcrtcbrata, al- 

 6, its ramifications. ,i i ,t /^ tti* i 



though those of J^ ishes are occa- 

 sionally destitute of them. The dark appearance which they 

 present is not due to opacity, but is simply an optical eifect, 

 dependent (like the blackness of air-bubbles in liquids) upon the 

 dispersion of the rays by the highly-refracting substance that 

 surrounds them (§ 98). The size and form of the laeunse difler 

 considerably in the several Classes of Vertebrata, and even in 

 some instances in the Orders ; so as to allow of the determination 

 of the tribe to which a bone belonged, by the microscopic exami- 

 nation of even a minute fragment of it (§ 453). The following 

 are the average dimensions of the lacunae, in characteristic ex- 

 amples drawn from the four principal classes, expressed in 

 fractions of an inch : — 



Long; Diameter. Short Diameter. 



Man, 1-1440 to 1-2400 1-4000 to 1-8000 



Ostrich, 1-1333 to 1-2250 1-5425 to 1-9650 



Turtle, 1-375 to 1-1150 1-4.500 to 1-5840 



Conger-eel, .... 1-550 to 1-1135 1-4500 to 1-8000 



The lacunse of Birds are thus dietinguished from those of Mam- 

 malg by their somewhat greater length and smaller breadth ; but 

 they differ still more in the remarkable tortuosity of the canali- 

 culi, which wind backwards and forwards in a very irregular 

 manner. There is an extraordinary increase in length in the 



