first layer (wall) :

S12 = (1/((R12+R21)*C11)) ;                                        link of the first layer with the second layer

S11 = -( (1/((RklS1+R11)*C11)) + S12 );                      basic equation of the first layer

second layer (wall) :

S21 = (1/((R12+R21)*C22)) ;                                        link of the second layer with the first layer

S22 = -( (1/((R22+Rkl1)*C22)) + S21 );                        basic equation of the second layer

first layer (body) :

T12 = (1/((Rt12+Rt21)*Ct11)) ;                                     link of the first layer with the centre

T11 = -( (1/((RklS1+Rt11)*Ct11)) + T12 );                    basic equation of the first layer

center (body) :

T21 =  (1/((Rt12+Rt21)*Ct22)) ;                                    link of the centre with the first layer

T22 = -( T21 );                                                                basic equation of the centre

link of the first layer of the wall with the first layer of the body:

SS1 = (1/((RklS1+R11+RklT1+Rt11)*C11)) ;

link of the first layer of the body with the first layer of the wall:

TT1 = (1/((RklS1+R11+RklT1+Rt11)*Ct11)) ;

Interfering influences in relation to the wall and the body:

B1 = q1*SS1*(RklT1+Rt11) ;

B2 = q1*TT1*(RklS1+R11) ;

Matrix:

yp = [

S11*y(1) + S12*y(2)  + SS1*y(3) + 0                + B1

S21*y(1) + S22*y(2)  + 0               + 0               + 0

TT1*y(1) + 0               + T11*y(3)   + T12*y(4)  + B2

0              + 0               + T21*y(3)   + T22*y(4)  + 0

] ;