72 OSTEOLOGY. 
which enter the surface of the shaft and epiphysis; but in addition there is a 
larger trunk which enters the diaphysis and reaches the medullary cavity. This is 
called the nutrient artery of the bone. The direction taken by this vessel varies in 
different bones. In the upper limb the artery runs downwards in the case of the 
humerus, and upwards in the radius and wna; in the lower limb the nutrient 
vessel of the femur is directed towards the upper extremity of the shaft, whilst 
in the tibia and fibula it follows a downward course. It is difficult to account for 
this difference in the arrangement of the vessels; but it has been pointed out that 
when all the joints are flexed, as in the position occupied by the fcetus in utero, 
the direction taken by the vessels is the same, and corresponds to a line passing 
from the head towards the tail-end of the embryo. Consequently, in the upper 
limb the vessels flow towards the elbow, whilst in the lower limb they pass from 
the knee. 
The attention of anatomists has long been directed to the elucidation of the laws 
which regulate bone-growth. Our pr esent knowledge of the subject may be briefly 
summarised in the following veneralisations :— 
1. In bones with a shaft and two epiphyses, the epiphysis towards’ which the 
nutrient artery is directed is the first to unite with the shaft. 
2. In bones with a shaft and two epiphyses, as a rule the epiphysis which com- 
mences to ossify latest unites soonest with the shaft. (The fibula is a notable ex- 
ception to this rule. See p. 231.) 
3. In bones with a shaft and one epiphysis the nutrient artery is directed 
towards the end of the bone which has no epiphysis. (This arrangement holds 
good in the case of the clavicle, the metacarpus, metatarsus, and phalanges.) 
4, When an epiphysis is ossified from more than one centre, coalescence takes 
place between the separate ossific nuclei before the epiphysis unites with the shaft. 
Highly suggestive, too, are the following propositions—That ossification first 
commences in the epiphysis which ultimately acquires the largest relative propor- 
tion to the rest of the bone, and that the ossification of the epiphysis is also 
correlated with its functional importance. In cases of long bones with only one 
epiphysis, the epiphysis is placed at the end of the bone where there is most 
movement. 
The veins which permeate the cancellous texture of the bone are large and thin- 
walled. They do not accompany the arteries, and, as a rule, in long bones they 
escape through large openings near the articular surface. In flat bones they occupy 
channels within the diploe, and drain into an adjacent sinus, or form communica- 
tions with the superficial veins of the scalp. 
The lymphatics are mainly periosteal, but enter the bone along wath the vessels 
and become perivascular. 
The nerves which accompany the arteries are probably destined for the supply 
of the coats of these vessels. Whether they end in the bony tissue or not is un-. 
known. 
THE VERTEBRAL COLUMN. 
The vertebral column (columna vertebralis) of man consists of thirty-three 
segments or vertebre, placed one on the top of the other. In the adult, certain of these 
vertebree have become fused together in the process of growth to form bones, the 
segmental arrangement of which is somewhat obscured, though even in their fully- 
developed condition sufficient evidence remains to demonstrate their compound 
nature. The vertebrxe so blended are termed the fixed or false vertebre, whilst 
those between which the osseous union has not taken place are described as the 
movable or true vertebre. This fusion of the vertebral segments is met with at 
either extremity of the vertebral column, more particularly below, where the column 
is modified to adapt it for union with the girdle of the lower limb, and also in 
the region of man’s degenerated caudal appendage. But a partial union of the 
vertebral segments also takes place above, between the two highest vertebre, im 
association with the mechanism necessary to provide for the movements of the 
head on the column. 
