3 88 OUTLINES OP EqUINE ANATOMY. 



extremities of the intercostal spaces over the external 

 surface of transversalis abdominis, forming remarkable 

 parallel white cords (falsely termed abdominal ribs). The 

 most ]3osterior of these are derived from the lumbar 

 nerves which resemble the dorsal, but, of course, do not 

 possess intercostal branches. 



The THORAX must now be opened, which may be done 

 by cutting through the superior and inferior extremities of 

 the ribs from the second to about the twelfth by means of 

 a saw, and removing the separated portion of the wall of the 

 chest by cutting through the first and twelfth intercostal 

 muscles. The pleura or serous lining membrane of the 

 chest is thus exposed, and since it is very transparent we 

 can see the structures it covers very plainly through it. 

 The thorax is the anterior cavity of the trunk. It is 

 pyramidal in form, its apex being anteriorly placed, its 

 base cut off obliquely from below upwards posteriorly. 

 Superiorly it is bounded by the under surfaces of the^ 

 bodies of the dorsal vertebrae, which centrally form a 

 prominent ridge, which anteriorly from the sixth vertebra 

 is bounded on either side by a prominent fleshy mass, the- 

 thoracic ])ortion of longus colli. On either side the roof of 

 the thorax is formed by the arch of the ribs, while the 

 straighter portion of them passes downwards to articulate 

 with the sternum through the medium of the costal carti- 

 lages, thus with the aid of the intercostal muscles form- 

 ing the sides of the thorax. Its floor is about one half- 

 the length of the roof, and is formed of the sternum with 

 its cartilages. Its apex is the space between the two first 

 ribs, through which important structures pass, i. e. longus 

 colli muscles (2), common carotid, axillary (2), vertebral 

 (2), inferior cervical (2), external pectoral (2), arteries with 

 the corresponding veins (jugular vein corresponds to 

 carotid artery), trachea, oesojDhagus, pneumogastric, recur- 

 rent, phrenic, gangliated cord of sympathetic nerves. 



The base of the thorax is formed by the anterior, coru 

 vex, surface of the diaphragm, the thin expanded muscle 

 which separates this cavity from the abdomen. The 



DIAPHRAGM presents centrally a heart-shaped tendi- 

 nous portion, the fibres of which are much interlaced, and 

 which, on the right side by separation of its fibres, forms 

 the foramen dextrum, by passing through which the 

 posterior vena cava gains the cavity of the chest, during 



