358 ANATOMY AND PHYSIOLOGY OF THE HORSE’S FOOT. 
each is united by transverse fibres in the form of a compara- 
tively thick membrane that is at first only applied against 
the tendon. As the latter, however, diminishes in substance 
and increases in width, this accessory sheath begins to adhere 
to it more and more closely, until behind the lower face of 
the navicular bone — the most vulnerable and severely tested 
portion of the tendon in some respects — the texture of both 
has become so intimately fused as to be inseparable; they 
are completely incorporated. There is no union along the 
borders of the tendon; this would be unnecessary, and each 
preserves its autonomy until the retrossal processes are 
gained, when tendinous and accessory fibres are alike in- 
serted into the inner faces of these, along with the fasciculi 
from the lateral cartilages. 
As has been justly remarked, the terminal expansion of 
the perforans tendon, when completed by these accessory 
fibrous structures, may be considered, with regard to the 
articulation of the foot, as a strong membranous ligament, 
attached above to the large pastern bone by the two lateral 
bands just indicated, and below by its attenuated termination 
to the plantar face of the pedal bone. Viewed in this 
manner, it will be seen that these powerful supplementary 
membranes, extending from the sides of the first phalanx to 
the retrossal processes of the os pedis, assume the office of a 
third pair of lateral ligaments by still further assuring the 
solidity existing between the bones of this region; but 
beyond this, they accomplish a perhaps more essential end 
in supporting and enhancing the action of the tendon at the 
instant when it is called upon to flex the foot, and in relieving 
it from strain when the weight of the limb and body is, either 
in movement or while in a standing posture, thrown upon it. 
With another apparatus to be hereafter described, this acces- 
sory fibrous membrane, in playing the part of a wide resisting 
brace, maintains the flexor tendon in its proper situation 
firmly against the lower face of the navicular bone. 
I have already mentioned that the tendons pass through 
sheaths or bursa that are lined by synovial membranes, 
which, with their secretion, ensure these cords easy and 
rapid movement in situations where they would otherwise 
meet with obstruction and injury, and I have alluded to that 
in connection with the deep flexor tendon and the navicular 
bone. In a healthy condition this synovial sac is, perhaps, 
less vascular than some of those immediately above it, and 
notably that behind the large and small pastern bones. In 
the limb of a two-year old colt which I succeeded in injecting 
most beautifully, this navicular bursa shows a smaller number 
