TRIANGULARIS STERNI. DIAPHRAGM. 243 



The subcostal or infracostal muscles are small bundles lying on the inner 

 aspect of the thoracic wall, close to the surface of the internal iuter- 

 costals, and chiefly in the neighbourhood of the angles of the ribs. They 

 follow the same direction as the internal iutercostal muscles, but their fibres 

 extend over two or three iutercostal spaces. They are most constant on 

 the lower ribs (see fig. 195.) 



The it iauyularis sternly a thin stratum of muscular and tendinous fibres 

 placed within the thorax, behind the costal cartilages, arises from the deep 

 surface of the ensiform process of the lower part of the body of the 

 sternum, and of the cartilages of one or two of the lower true ribs. Its 

 fibres pass outward and upwards in a diverging manner, the lowest being 

 horizontal, the middle oblique, and the upper becoming more and more 

 nearly vertical ; and are inserted by separate slips into the cartilages of the 

 true ribs from the sixth to the second inclusive, on the lower border and 

 inner surface of each, at the junction with the bony part. At the lower 

 margin the fibres are in the same plane with those of the transversalis 

 abdoininis, of which this muscle is a continuation upwards. 



The triangularis sterni is subject to much variation as to its extent and points of 

 attachment in different bodies, and even on the opposite sides of the same body. 

 The internal mammary artery and veins pass between its anterior surface and the 

 costal cartilages. 



Occasional Muscle. The rectus sternalis, or sternalis brutorwm, is an elongated 

 muscle, of nearly the same length as the sternum, frequently seen lying in front and 

 parallel to the outer margin of that bone, and over the inner part of the pectoral muscle. 

 It is fleshy in the middle and tendinous at both ends it is attached superiorly to the 

 sternum in connection with the tendon of the sterno-mastoid, and inferiorly it is 

 connected with the rectus abdominis muscle. It is rarely present on both sides. It 

 represents a muscle which is constant in some animals. 



THE DIAPHRAGM. 



The diaphragm, phren, septum transversum, or midriff, forms a musculo- 

 tendinous partition between the abdominal and thoracic cavities. It 

 consists of muscular fibres which arch upwards as they converge from the 

 circumference of the visceral cavity to a tendinous structure in the centre, 

 and it is perforated by the various structures which pass from the thorax to 

 the abdomen. 



The fibres arise from the bodies of several of the upper lumbar vertebrae 

 by two thick crura ; from two arches on each side external to the bodies of 

 the vertebras, called ligarnenta arcuata ; and from the ensiform cartilage 

 and the cartilages of the six lower ribs. 



a. The crura arise by tendinous fibres more or less distinctly aggregated 

 into two bundles which are attached over a varying extent of surface. On 

 the right side they are connected with the first, second, and third lumbar 

 vertebrae, and the interposed fibro- cartilages, or sometimes with the second, 

 third, and fourth ; on the left side, the attachment is shorter by the breadth 

 of one vertebra. The tendons of both crura curve forwards and upwards 

 so as to enclose the aorta in an arch between them and the bodies of the 

 vertebras ; their inner margins are united behind, so that they form a 

 nearly complete fibrous ring round that vessel. The muscular fibres of the 

 crura, springing from those tendons in thick bundles, on each side of 

 the aorta, diverge as they pass upwards to the central aponeurosis. The 

 innermost fibres on each side decussate with those of the side opposite, those 



