knob is turned counter clockwise as shown in fig6 b the pin 451 on the catch block 45 will be driven along the slot 412 to cause the catch block 45 to move leftwards so as to have the pointed pawl 452 disengaged from the ratchet wheel 44 in that case the auxiliary instrument tray 4 can be turned freely to a suitable position desired then the adjusting knob 41 is turned to a position as shown in fig6 a to have the pointed pawl 452 engaged with one tooth of the ratchet wheel 44 whereby the auxiliary instrument tray 4 is fixed at a given position fig5 a illustrates that both ends of the auxiliary instrument tray are provided with the aforesaid structure as shown in fig5 b referring to fig1 a there is a board 222 being attached to a support arm 27 as shown in fig2 which is attached to the base 19 the angle adjusting knob 21 has a sliding slot 212 for receiving the pin 251 of a catch member 25 a shaft 23 is used to couple the ratchet wheel 24 and the catch member 25 together the pawl 252 of the catch member 25 is mounted inside a recess of the block 26 and is loaded with a.