THORAX AND ABDOMEN OF THE HORSE 11 



cavity of the thorax is not so spacious as an examination of the skeleton 

 would lead one to imagine. This may be realised by reference to 

 Figs. 39-42, in which it is indicated that, at the end of expiration, 

 the curve of the diaphragm, about the median plane of the body 

 reaches the level of the sixth rib. 



The undissected wall of the thorax may be described as containing 

 the following six layers : — (1) The skin. (2) The cutaneous muscle and 

 fascia. (3) Several strata of muscles applied to the outer surface of the 

 ribs, vertebrse and sternum. (4) A bony skeleton consisting of the 

 thoracic vertebrae, the ribs (including bony and cartilaginous segments), 

 and the sternum. Between the ribs are the intercostal sjmces (spatia 

 iutercostales) occupied by the intercostal muscles. (5) The endothor- 

 acic fascia, with which are associated the longus colli and transverse 

 thoracic muscles. (6) The pleura. 



The entrance to the thorax (apertura thoracis cranialis), ovoid in 

 outline and flattened laterally, is bounded by the first thoracic vertebra, 

 the first pair of ribs and the cranial end of the sternum. It is occupied 

 by the longus colli muscle, trachea, oesophagus, nerves, large blood 

 vessels proceeding to and from the neck and the thoracic limbs, lymphatic 

 vessels and lymph glands, and (in the young) the thymus. 



The opposite extremity of the thoracic cavity (apertura thoracis 

 caudalis) is bounded by the last thoracic vertebra and the costal arch, 

 and is closed by the diaphragm. 



It is convenient to consider that the thorax po.ssesses a dorsal, a 

 ventral and right and left lateral walls, though the demarcation of these 

 is largely arbitrary. The dorsal wall may be held to correspond to the 

 thoracic vertebrae and the ribs as far as their angles, with the ligaments 

 and muscles connected with these bones. The ventral wall is formed 

 by the sternum and the cartilaginous segments of the sternal ribs, with 

 their associated muscles and ligaments. The lateral wails consist of 

 the bony segments of the sternal ribs from their angles onwards, the 

 corresponding part of the asternal ribs and their cartilaginous segments 

 and the intercostal muscles. The limits of the lateral wall of the thorax, 

 as distinct from the wall of the abdomen, are marked by the border of 

 the last rib and the gently curved line, the costal arch (arcus costalis), 

 with a concavity looking in a dorsal and cranial direction, formed by 

 the overlapping cartilages of the asternal ribs. 



Although the cavity of the thorax has been described above as 

 having the form of a flattened cone with a sloping base, this description 

 requires amplification, as can best be done by a consideration of the 

 various diameters of the cavity. Because the distance between the 

 sternum and the vertebral column increases from the level of the first 



