IO22 



VEETEBEATED ANIMALS 



microscopic examination of even a minute fragment of it. The 

 following are the average dimensions of the lacuna?, in characteristic 

 examples drawn from four principal divisions, expressed in fractions 

 of an inch : 



Man . 



Ostrich 

 Turtle . 

 Conger-eel 



Long Diameter 

 1-1440 to 1-2400 

 1-1333 1-2250 

 1-375 1-1150 

 1-550 , 1-1135 



Short Diameter 

 1-4000 to 1-8000 

 1-5425 1-9050 

 1-4500 ,. 1-5840 

 1-4500 1-8000 



The lacume of birds are thus distinguished from those of mmn- 

 mals 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 vcrv 

 irregular manner. There is an 

 extraordinary increase in length 

 in the lacunae of reptiles, with- 

 out a corresponding increase in 

 breadth ; and this is also seen 

 in some fishes, though in ge- 

 neral the lacuna? of the latter 

 are remarkable for their angularity of form and the fewness of their 

 radiations, as shown in fig. 75o, which represents the lacunae and 

 canaliculi in the bony scale of the Lepidosteus (' bony pike ' of the 

 North American lakes and rivers), with which the bones of its in- 

 ternal skeleton perfectly agree in structure. The dimensions of the 

 lacume in any bone do not bear any relation to the size of the animal 



FIG. 752. Lacuiire of osseous substance 

 a, central cavity ; l>, its ramifications. 



FIG. 753. Section of the bony scale of Lr/i/,/,i^/,-:is : a, show- 

 ing the regular distribution of the lacunae and of the connecting 

 canaliculi ; b, small portion more highly magnified. 



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

 between their size in the enormous extinct Iguanodon and ill the 

 smallest lizard now inhabiting the earth. Hut they bear a close rela- 

 tion to the size of the blood-corpuscles in the several classes; and 

 this relation is particularly obvious in the perennibraiichiate ' 

 I'.atrachia. the extraordinarily large size of whose blood-corpuscles 

 will be present 1\ not iced. 



