110 



LOCOMOTION. 



[CHAP. v. 



imagined to be solid corpuscles (a name still commonly applied to 

 them), and the lines radiating from them to be branching threads 

 of the earthy constituent of bone. They may be proved in many 

 ways, however, to be real excavations in the tissue. With a suffi- 

 ciently high power their opposite walls can be distinctly seen, as 

 well as their hollow interior; but the most conclusive evidence 

 lies in our being able to fill them with fluid. If a dry section of 

 bone, in which they are very apparent, be moistened with oil of 

 turpentine while in the field of the microscope, the course of this 

 penetrating material can be witnessed, as it advances into the tissue, 

 It is seen to run quickly along the pores from the Haversian canals, 

 and from the surface of the specimen, where they have been cut 

 across. Having entered a lacuna, it suddenly extends along the 

 pores radiating from it, and, through these, reaches other lacunae ; 

 rendering the tissue transparent by filling up its vacuities. In parts 

 where air has previously occupied the vacant spaces, and the tur- 

 pentine cannot displace it, the characteristic appearance of minute 

 bubbles is often present. 



The lacunae of osseous tissue, if examined extensively in the ver- 

 tebrate class, are found of very various shapes : sometimes scarcely 

 to be distinguished from the pores, of which they are simple fusi- 

 form dilatations ; at other times large and bulky, and forming the 

 point of junction of a great multitude of pores. Mr. Tomes has 

 allowed us to represent the principal varieties which he has met 



Fig. 24. 



d,' 



Form of various Lacunae, and their pores : a. Simple irregular cavities, without pores ; from 

 an ossification of the pleura : 6. from healthy bone of the human subject. 6'. One of the outer 

 lacunae of an Haversian system, with the pores all tending down towards the H. canal, c. Other 

 forms from human hone, shewing the lateral connecting pores. 



d. From the Boa. External lacuna 1 , of an H. system, with unusually large pores dipping to- 

 wards the vascular surface, d'. Cavity intermediate between a lacuna and a pore. e. Another 

 variety from the same reptile. From Mr. Tomes. 



