VERTEBRATED ANIMALS. 



257 



653, The minute cavities or lacunce (sometimes, but erroneously 

 termed ' bone-corpuscles/ as if they were solid bodies), from which the 

 canaliculi proceed (Fig. 441), are highly characteristic of the true osse- 

 ous structure; being never deficient in the minutest parts of the bones 

 of the higher Vertebrata, although those of Fishes are occasionally des- 

 titute of them. The dark appearance which they present in sections of 

 a dried bone is not due to opacity, but is simply an optical effect, depen- 

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

 of the rays by the highly refracting substance that surrounds them 

 ( 153). The size and form of the lacunae differ considerably in the sev- 

 eral 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 examination of even a minute fragment of it ( 705). 

 The following are the average dimensions of the lacunae, in characteris- 

 tic examples drawn from the four principal Classes expressed in frac- 

 tions of an inch : 



Long Diameter. 



Man 1-1440 to 1-2400 



Ostrich 1-1 333 to 1-2250 



Turtle 1-375 to 1-1150 



Conger-eel 1-550 to 1-1135 



FIG. 442. 



Short Diameter. 

 1-4000 to 1-8000 

 1-5425 to 1-9650 

 1-4500 to 1-5840 

 1-4500 to 1-8000 



Section of the Bony Scale of Lepidosteus :<i. showing the regular distribution of the lacunge 

 and of the connecting canaliculi ; 6, small portion more highly magnified. 



The lacunae of Birds are thus distinguished from those of Mammals by 

 their somewhat greater length and smaller breadth; but they differ still 

 more in the remarkable tortuosity of their canaliculi, which wind back- 

 wards and forwards in a very irregular manner. There is an extraordi- 

 nary increase in length in the lacunae of Reptiles, without a correspon ding- 

 increase in breadth; and this also seen in some Fishes, though in general 

 the lacuna? of the latter are remarkable for their angularity of form 

 and the fewness of their radiations, as shown in Fig. 442, which rep- 

 resents the lacunae and canaliculi in the bony scale of the Lepidosteus 

 ('bony pike 7 of the North American lakes and rivers), with which the 

 bones of its internal skeleton perfectly agree in structure. The dimen- 

 sions of the lacunae in any bone do not bear any relation to the size of 

 the animal to which it belonged ; thus there is little or no perceptible 

 difference between their size in the enormous extinct Iguanodon, and in 

 the smallest Lizard now inhabiting the earth. But they bear a close rela- 

 tion to the size of the Blood-corpuscles in the several Classes; and this rela- 

 tion is particularly obvious in the ' perennibranchiate ' Batrachia, the ex- 

 traordinary size of whose blood-corpuscles will be presently noticed 

 (665): 



17 



