182 ANATOMY AND PHYSIOLOGY OF THE HORS E , S FOOT. 
extent as a diverticulum to the mass of blood sent to or re- 
turning from the foot, results from the confluence of all the 
canals proceeding from both surfaces, often uniting many 
times in their converging course, until they reach the grand 
curvature of the sinus, which they all enter by a limited 
number of openings. The sinus itself continues to bend 
backwards, and finally gains the exterior of the bone by two 
orifices — the plantar foramina — some distance apart, situated 
beneath the large articulating surface towards the posterior 
portion of the arch of the sole, within the wings of the os pedis 
(fig. 5, c). In this locality, as before remarked, the dense bony 
tissue is most abundant and most skilfully disposed to pre- 
serve intact this important cavity or sinus : sending out 
perpendicular prolongations, like so many resisting pillars, 
to give additional strength to the whole foundation, and 
fortifying it against the superimposed weight, which is ever 
tending to crush the articular or upper, and lower or ground, 
surfaces together. Not unfrequently we find a more com- 
plicated and wonderful arrangement of these canals than is 
represented in figure 5. Some show a very curious and 
intricate disposition, indicated by M. H. Bouley, in which 
there is a secondary sinus, with very numerous radiating 
branches that frequently anastomose with each other before 
opening into the circumflex artery. 
If we remove the anterior wall of the os pedis from below 
the pyramidal eminence, the existence of this series of per- 
pendicular or descending canals will he again revealed, as 
they communicate with the semilunar sinus. 
As I have already said, to study the conformation and 
structure of the chief bone of the foot, one of a compara- 
tively early age, say from four to six years old, should be 
obtained ; as generally with age, no less than through the 
effects of continued shoeing, labour, mismanagement, and 
disease, it becomes considerably and prejudicially altered. 
The inferior margin, or what is frequently termed the “ bear- 
ing surface,” becomes rounded, the toe portion especially is 
sometimes so much absorbed as to appear turned upwards, 
and the two wings, instead of standing on the same plane, 
also bend upwards or even downwards, while morbid depo- 
sitions of osseous matter are frequently observed on various 
parts of the surface. 
In a healthy well-formed bone the whole of the lower or 
convex margin, from toe to heels rests pretty evenly on the 
same level, and the toe does not bend upwards to a greater 
or less extent, as is sometimes represented in drawings of 
this ray. The basilar process also submits to various altera- 
