INTIMATE STRUCTUEE. 255 



5, 5, right ventricle ; 6, left ventricle ; 6', apex of the heart : the white line outside the 

 heart is intended to indicate the external pericardium, as if the anterior half were removed 

 by a transverse incision ; 7, 7, vena cava superior ; 8, 8, internal jugular reins ; 9, 9, 

 subclavian veins, joining the jugular ; 9, 7, 9, innominate veins ; the right rising behind 

 the sterno-clavicular articulation, the left crossing obliquely behind the upper half of the 

 manubrium. The position of the first parts of the innominate artery, left carotid and left 

 subclavian arteries, is indicated behind and below this vein ; 9', 9', outer part of the sub- 

 clavian arteries. It is to be observed that in this figure the attachment of the sixth costal 

 cartilage to the sternum is represented a little too high. 



The right vmtride extends from above do^vu from the third to the 

 sixth cartilages on the left side.* The conus arteriosus is its most 

 projecting part, being uncovered bv lung. The ai/rict'Jo-vcntrkidar 

 side US corresponds with a line drawn obliquely upAvards from near the 

 sternal end of the 6th costal cartilage on the right side, to the Srd car- 

 tilage on the left. The rounded margin formed by the left ventricle 

 extends on the left side from the 3rd cartilage to a point in the fifth 

 space two inches vertically below the nipple. The sliarp margin formed 

 by the right ventricle passes from the sternal end of the Gth cartilage on 

 the right, and crosses behind the 7th right cartilage, the ensiform (at 

 its upper third), and the 7th left cartilage, to meet the other margin 

 at the a]5ex. 



The apex of tlielicart (fig. 174, C) is situated about ?A inches to the 

 left of the middle line, in the fifth intercostal space, close to the upper 

 margin of the sixth rib. 



The auricido-ventricidar openings lie slightly to the right of the line 

 of the auriculo-ventricular sulcus. The tricusjnd orifice lies behind the 

 lower fourth of the sternum, its upper border being on a level with the 

 fourth cartilages. The left auricido-vcntricidar opening extends from 

 the 3rd space to the 5th cartilage behind the left half of the sternum. 

 The orijice of the indmonarg arterg is placed immediately to the left 

 of the sternum, behind the edge of that lione and the 3rd cartilage ; 

 the aortic orifice, also partly behind the left half of the sternum, is on a 

 slightly lower level than the orifice of the pulmonary artery (being 

 opposite the lower part of the 3rd cartilage and the 3rd intercostal 

 space), and is covered by it in one-fourth its diameter. The aortic 

 orifice is exactly behind the posterior wall of the conus arteriosus. 



INTIMATE STRUCTURE OF THE HEART. 



The substance of the heart consists chiefly of muscular tissue ; but 

 besides this a certain amount of fibrous and fibro-cartilaginous tissue 

 is met with, collected principally at the base of the ventricles around 

 the ventricular orifices. 



Pibro-cartilage and fibrous tissue. — In the angle between the 

 aortic and the two auriculo-ventricular openings (see fig. 171, close 

 to 8) a fibro-cartilaginous mass is found, which in some animals, as the 

 ox and elephant, is bony, and is known as the on cordis. From this 

 central fibro-cartilage processes pass in various directions. One of these, 

 extending downwards to meet the fleshy septum of the ventricles, 

 separates the left ventricle from the right auricle, forming the right 

 boundary of the aortic vestibule. These processes form the bases of 

 what have been described by authors as the fibrous or tendinous rings of 

 the auriculo-ventricular and arterial openings. The fibrous tissue of 

 these rings is continuous with that which is found in the segments 



* These are sometimes termed the " cardiac cartilages." 



