114 
MESSRS. J. TOMES AND C. DE MORGAN ON THE 
they are even less constant. When present they appear to indicate that the bone 
is nearly stationary in its growth as regards thickness, for in the fast-growmg bones 
of young animals these laminae are entirely absent, while in the bones of adults they 
are usually well-developed in some parts. In number they seem subject to consider- 
able variation, and not unfrequently are arranged in different concentric series with 
three or four laminae in each division, separated from the adjacent series by a broad 
and transparent lamina, suggesting the idea of periods of greater activity of growth, 
alternating with others of less activity. Like those of the Haversian systems, the 
circumferential laminae are perforated by Haversian spaces and systems (Plate ^ II. 
fig. 10), and in many examples to such an extent that they are recognized only as 
interstitial laminae ; but their uniform direction parallel with the surface of the bone 
will satisfactorily indicate their previous existence as continuous circumferential 
laminae. In this part of a bone we may sometimes discover a peculiarity of character, 
in the presence of Haversian systems, consisting of two or three laminae, or of even 
a single lamina only, so small has been the preceding Haversian space. 
In the progress of development canals are sometimes formed which proceed from 
the surface of the bone into its substance, and around these the circumferential 
laminae are inflected, thereby forming systems, the laminae of which are continuous 
with those of the surface of the bone. On the other hand, Haversian spaces not 
unfrequently occur which are continued from the surface towards the centre of the 
bone, cutting the circumferential laminae abruptly across j systems are subsequently 
developed in these spaces ; but in such instances the laminae of the systems are not 
continuous with those of the surface (the latter appearance is shown in Plate VII. 
fig. 10). , 1 j 1, 
In young rapidly growing bone the circumferential laminae are replaced by a 
series, which, from the character of their course, might be termed the undulating 
lamina The bone in its growth sends off outrunning processes between those vessels 
of the periosteum which lie nearest the surface of the bone. These processes are 
formed of reduplicated laminae which are continued from process to process, follow- 
ing the undulating surface of the bone for a considerable distance before they are 
lost The growing processes as they increase in size and length arch over towards 
each other by the dilatation or bifurcation of the outer extremities. The salient 
points so formed come into contact and unite, thereby enclosing the vessels. The 
spaces enclosed in the manner described become the seat of Haversian systems. 
Hence we have at this period of growth, bone composed of Haversian systems with 
intervening undulating laminm, a character by which the observer cannot fail to 
recognise young growing bone as differing from that of adult animals. Over the 
points where the processes have met and united, laminae are extended which are 
continued outwards to form similar and more external processes ; these in their turn 
become united and inclose spaces. This mode of increase by the development of 
outgrowing processes is repeated again and again until the rate of growth becomes 
