Thorax 



49 



aperture, bounded by the cartilages of the false ribs, the last ribs, and the twelfth dorsal vertebra, 

 is in a plane sloping in the opposite direction at least, in front of the tenth rib. 



Looking into the cavity, the bodies of the vertebrae are seen to make a prominent 

 median dorsal projection into it, so that a transverse section would give a reniform 

 shape to the cavity. This vertebral ridge, together with the backward direction of 

 the ribs in the region of their necks, leads to the existence on each side of it of a deep 

 and broad pulmonary groove that is filled by the thick posterior border of the lungs. 



On their outer side the ribs are covered for the most part by large flat muscles 

 which lie on them. Fig. 39 shows these relations : the pectorals have been in great 

 part removed on the one hand, and on the other the scapula and Subscapularis, with 

 the exception of a strip of their inner parts, have been cut away : thus the Serratus 

 magnus is exposed, covering the greater part of the upper eight ribs, with the External 

 Oblique below this, covering the lower eight and the cartilages. The Latissimus dorsi 

 lies on those ribs that are not covered by Serratus magnus above the External Oblique ; 

 its origin is by digitations in series with those of the Serratus, but being more vertically 

 directed it passes into a more superficial plane, first covering the ribs below the Serratus. 

 The cartilages of the false ribs are covered by External Oblique except in front, where, 

 the Rectus lies on them and sepa- 

 rates them from the aponeurosis of 

 the oblique muscle ; here the Pec- 

 toralis major obtains some origin 

 from the aponeurosis. 



We thus see that, so far as the 

 ribs are concerned, three large 

 muscles cover them nearly alto- 

 gether, and arise from them. Ex- 

 amine the line of the " anterior 



angles " in the articulated skeleton, FlG 40 ._ An terior angle of a middle rib ; the arrows show 



and it is evident that this COrre- the direction of fibres of External Oblique, some arising 



j .,, , ,. - . , ,. . from the rib and making the lower surface, others 



spends With the line Of interdlgl- coming from the bone above and thus pressing in 



tation of External Oblique with the front P art of the rib - Serratus magnus arises 



j T from the posterior area, runs in the direction ol the 



herratUS magnus and LatlSSiniUS arrow s.M. and moulds the rib lying under it. 



dorsi : in fact the bend here is 



apparently due to the pressure of these flat muscles, and if a well-marked " anterior 



angle" is examined three surfaces can be found meeting on it, as shown in Fig. 40. 



The lower surface is for the origin of a digitation of the External Oblique, the 



area in front of this is covered by the digitation of the same muscle arising from 



the rib next above, while the posterior area is covered by and gives origin to Serratus 



magnus. 



Above the fifth rib the definite " anterior angle " is not so clear, but the ribs show 

 signs, in some cases, of the origin of Pectoralis minor in front of Serratus magnus. 



The digitations of Serratus magnus from the two upper ribs, on the most in-turned 

 part of the wall, support the axillary sheath in the top of the axilla. Immediately 

 in front of these are the pectoral structures of the anterior wall of the axilla, lying on 

 these ribs. 



Further back, the first rib is above the level of the Serratus, and is crossed here by 

 the Scalenus posticus, which goes to the second rib under cover of the top of Serratus 

 magnus. 



Though covered by these large muscles, most of the ribs can be felt on the side of the 



F.A. 4 



