And the possibility of losing at any moment perhaps the most valuable part of your outfit is a constant and severe strain on your mind.

Nobody except those who have travelled in the Mexican mountains can understand and appreciate the difficulties and anxieties attending such a journey.

Not only the animals themselves, but everything they carry is vital to the success of the expedition, and there is always a danger that, for instance, your camera and photographic outfit, and the priceless collection of negatives already taken, may roll down a precipice.
A mule with its bulky pack is, to a certain extent, helpless on these narrow mountain trails.

Old and experienced animals often manoeuvre their packs with a cleverness that is almost human: yet, whenever a mule runs accidentally against some projection, or its foot slips, the poor beast invariably loses its balance, and over it goes, down the hill with ever-increasing velocity.
On one occasion I heard a noise coming from above without being at first able to discern what caused it.

A few stones came tumbling down, and were presently followed by a donkey, pack and all, turning over and over with astounding speed.