CONTENTS OF THE THORAX, 407 
the posterior part of the plantar surface of the os pedis. Ac#on—-to 
extend the hock and to flex the fetlock and pasterns, 
FLEXOR PEDIS ACCESSORIUS lies on the postero-internal part of the leg. 
Origin—tfrom the supero-external part of the tibia and side of the fibula. 
Its znsertion is blended with the tendon of the flexor pedis. 
CHAPTER XXI. 
THE THORACIC ORGANS AND THEIR APPENDAGES, 
CONTENTS OF THE THORAX—THE BLOOD—GENERAL PLAN OF THE CIRCULATION—THE 
HEART AND ARTERIES—THE VEINS—PHYSIOLOGY OF RESPIRATION—-MECHANISM OF 
THE PULMONARY APPARATUS—THE NASAL ORIFICES AND CAVITIES—THE LARYNX 
—THE TRACHEA AND BRONCHI—THE LUNGS—PULMONARY GLANDS. 
CONTENTS OF THE THORAX. 
THE THORAX, OR CHEST, is that cavity formed by the bodies of the dorsal 
vertebra superiorly ; by the ribs and their cartilages with the connecting 
muscles laterally ; by the sternum inferiorly ; by the diaphragm poste- 
riorly ; and by the inner margins of the first ribs and body of the first 
dorsal vertebra anteriorly. It contains the central parts of the important 
organs of circulation and respiration, and gives passage to the cesophagus, 
as it connects the pharynx with the 
stomach. As these lie within it, they 
are allowed to play freely in per- 
forming their functions, by being en- 
veloped by smooth serous membranes, 
called the pleura and the pericardium, 
the latter being also protected by 
a fibrous layer, A section of the 
thorax, as shown in the plan, Fig. 1, p 
will give some idea of the relative 
situation of these organs and their 
investments, as well as of the shape 
of the cavity itself in this direction. 
The heart is shown at A, lying be- 
tween the two bags of the pleura, in 
the space called the mediastinum. The 
lungs are shown at B B, covered by 
a fine serous membrane (HH), pleura 
Fic. 1.—Sectionan Puan oF TAGRAX AND I7¢ 
pulmonalis, except at their roots, ConrEeNTS (THROUGH THE 91:2TH-PLACK). 
where the air tubes and blood vessels ree 
pass into their substance. This por- —— F. Walls of the thorax. 
a 4 ° : . G. Pleura costalis. 
tion of the pleura is continuous with Fe Ee Plea palmonalis 
the serous membrane lining the ribs 
(GG, pleura costalis), which thus allows them to expand and contract 
freely, by allowing one surface to glide against the other. Thus, the 
pleura on each side covering the lungs, and reflected thence to the 
inside of the ribs, and the thoracic side of the diaphragm, forms a 
shut sac of bag, which in the natural state contains only sufficient, serum 
