THOEAX AND ABDOMEN OF THE HORSE 145 



by rootlets from the last two or three thoracic sympathetic ganglia. It 

 enters the abdomen by crossing the lumbo-costal arch in company with 

 the greater nerve, and ends by joining the nerve plexus about the hilus 

 of the kidney. 



The diaphragm (Diaphragma). — The diaphragm ^ forms a dome- 

 shaped, muscular and tendinous partition between the cavities of the 

 thorax and abdomen. Viewed from the abdominal side, it is concave in 

 all directions, and largely covered by peritoneum. The thoracic surface 

 is correspondingly convex, and is invested by the pleurae. The parti- 

 tion, as a whole, has a marked slope in a ventral and cranial direction 

 from the sixteenth or seventeenth thoracic vertebra to the base of the 

 xiphoid process of the sternum. The vertex of the dome of the diaphragm 

 occurs (during expiration) on a level with the seventh rib about the 

 junction of the dorsal and middle thirds of the vertical diameter of the 

 thoracic cavity. 



The diaphragm may be described as consisting of a tendinous centre 

 around which are grouped lumbar, costal and sternal muscular portions. 



Centi'um tendineum. — The tendinous centre is scarcely more than 

 2 mm. in thickness, and consequently the abdominal and thoracic 

 viscera in contact with its two surfaces are very close to each other. In 

 outline the tendinous centre resembles the conventional heart of play- 

 ing-cards, with the truncated and indented base dorsal in position. The 

 tendinous fibres forming the centre are chiefly radiate in disposition ; 

 but there are also interlacing fibres, especially noticeable about the 

 opening for the caudal vena cava. 



Pars lu'nibalis. — The lumbar part of the diaphragm consists of two 

 strong muscular crura (crura diaphragmatis), of which the right is by 

 far the larger and stronger. The right crus arises b}^ a flattened tendon 

 from the ventral longitudinal ligament on the bodies of the first four 

 (possibly five) lumbar and the last thoracic vertebra?. The muscular 

 fibres succeeding the tendon run in a ventral and cranial direction, in 

 the middle line of the body, to end diverging in the tendinous centre. 

 A slit-like opening, through which the oesophagus passes, separates the 

 fibres into two unequal bundles. 



The slenderer left crus is separated from the right by an opening 



occupied by the aorta. Its fleshy fibres continue a tendon that is 



attached to the ventral longitudinal ligament on the bodies of the first 



two himbar vertebrae. They terminate in the tendinous centre after a 



course to the left of the median plane. Some of the fibres of the two 



crura intermingle ventral to the opening for the aorta. 



J Std (dia) [Gr.], across ; <ppdy/xa (phragma) [Gr.], a wall. A partition wall. 

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