CHAP, v.] THE CATS MUSCLES. 141 



ribs. The intercostal nerves and vessels arc interposed between the 

 two sets of intercostal muscles (c). 



The triangularis steriii is the name given to a layer of muscle 

 which diverges forwards to the cartilages of the ribs from the deep 

 (inner) surface of the sternum (Fig. 78, ffi). 



The diaphragin is (as has been before mentioned) a very impor- 

 tant partly fibrous, partly muscular structure, which separates the 

 thoracic and abdominal cavities. It forms a partition convex 

 forwards and concave backwards, and is attached to the xiphoid 

 cartilage, to several ribs, to the centra of the lumbar vertebrae, and 

 to the aponeurosis, which invests the quadratus lumborum and 

 psoas muscles.* At its circumference the diaphragm is muscular ; 

 its central portion is tendinous. It is perforated for the passage of 

 certain organs — namelj^ for the oesophagus and for two great blood- 

 vessels called aorta and inferior vena cava. 



The muscles just enumerated aid the process of respiration as 

 follows : — 



Apart from the elasticity of the substance of the lungs, respiration 

 is effected by the successive enlargement and contraction of the 

 thoracic cavity. We have seen that the ribs are movably articu- 

 lated to the vertebral column. They can be drawn forwards (and 

 the cavity of the thorax can be in consequence enlarged) by the 

 external intercostals and levatorcs costarum, but the main agent in 

 this process is the contraction of the muscular fibres of the diaphragm 

 resulting in a diminution of its convexity. Air rushes in to fiU the 

 thus enlarging cavity, and we thus have Inspiration. Expiration is 

 effected by the dramug backwards of the ribs by the internal inter- 

 costals and by the relaxation of the contraction of the fibres of the 

 diaphragm, which in consequence resumes its more convex shape, 

 and so contracts the thoracic cavity, a process further aided by the 

 contraction of the muscles of the belly, the external and internal 

 oblique, the rectus and transversalis. 



The abdominal region of the body is invested by three great sheets 

 of muscle and membrane, which enclose and support the abdominal 

 ^'iscera — and tend by their contraction to expel the contents of such 

 viscera and, as just said, indirectly to aid in respiration. 



The first of these muscles, the external oblique, arises by eight 

 digitations from the eight hindmost ribs and from the lumbar fascia. 

 Its fibres pass obliquely downwards, backwards and inwards (towards 

 the mid- ventral line,) to be inserted by muscle and membrane into 

 the brim of the pelvis — part of the membrane (or aponeurosis,) 

 di^'ides into the "external" and "internal" tendons. Between 

 these tendons (of which the internal is the stronger,) an aperture is 

 left called the " external abdominal ring," (through which certain 

 structures pass,) bounded in front by what is called Poupart's 

 ligament. 



This so-called "ligament " is really but a band of delicate fascia, 



* Tliese are described amonast the muscles of the hind- limb. 



