CHAP.V.] LACUNA OF BONE. 113 



the cylindrical form, they often present an oval, or even a very irre- 

 gular, figure, on a section ; their close package having modified 

 their form. The periosteal and medullary layers are also well seen 

 on the same section, the latter curving inwards to constitute the 

 walls of the cancelli. These two layers are of very irregular thick- 

 ness, as the Haversian rods encroach on them unequally (fig. 25). 



On a further examination of such a section, with a sufficient mag- 

 nifying power, we observe the lacunae of the periosteal and medul- 

 lary layers facing those surfaces, and their pores opening upon them; 

 while the lacuna? of each Haversian layer all face the corresponding 

 canal, and their pores radiate from it (fig. 26, and the previous fig. 

 23, more highly magnified) . The Fig 2G 



Iacuna3 facing the Haversian sur- 

 face are generally curved concen- 

 trically with it. They are more 

 numerous, and their pores more 

 abundant, on the side where there 

 is most osseous substance, and 

 where it consequently extends 

 furthest from the source of nutri- 

 ment, the Haversian vessel. The 

 reason of the want of proportion 

 between the width of the canals V'r'jjj^ . '" ' --''' J 



and the thickness of their re- ^ t/^^' 



% ' vX".*^'' 



spective osseous walls, appears to 



be this, that the larger Canals Part of the p repara tion represented in the last 

 trjiYiermt vpepl +r rkfTiAi* rnrf figure, more highly magnified: she wing the pnc-k- 



transmit vessels to otner parts, age of the Haversian systems, and also the light 



Vp<iirlpi prmtfnniTur flnno wVnfli spaces between neighbouring ones. The system, 



> Containing tnOSC WniCn a>appcarsto flll up an interval between the-others. 



nnnriili flipir nwn ImrAr Tiilp Tne laciuiso are seen facing the I lavcrshm canals, 



LSI! tlieir OW. laer Willie a nd their pores taking a general radiating direction. 



nf fhpm nrn nn rlrmlvf- in n At 5 an irregular lacuna communicates with the 

 n are, n LDt, m a poresof three contiguous systems. 



great measure channels for veins. 



The outer lacunae belonging to an Haversian canal sometimes 

 send out pores to anastomose with those of the neighbouring rods ; 

 but this seems to happen chiefly where the contiguous rods have 

 just sprung from a common stock. Occasionally, also, lacuna of 

 irregular shape (as at s, fig. 26) lie in the interval of two or more 

 rods, and communicate with lacunse of all of them ; but, in general, 

 the outermost pores of the extreme lacunse droop back on all sides 

 (fig. 24, b 1 ', d), and re-enter the penultimate series of their own rod. 



Owing to this arrangement, there always appears a transparent 

 interval between contiguous rods ; the pores and lacunae not existing 

 there to intercept the passage of the light (fig. 26). This is a 



VOL. i. i 



