referring now to the drawings , preferred embodiments of the medical bed and the device for retaining a patient in a predetermined posture on the bed according to the present invention will be described . fig1 is an explanatory view generally showing an embodiment of the medical bed according to the present invention . the medical bed 1 comprises a rectangular bed plate 3 including a flat board 4 which is fixed on a bedstead 5 having leg portions 51 . the bed plate 3 includes an elastic pad 7 which is made of elastic material and adhered to an upper surface of the flat board 4 to cover it for absorbing shock , so that a patient can comfortably lie with no discomfort on the bed plate 3 for medical treatment . the elastic material is preferably selected from sponge , gum rubber , plastic foam and the like . if necessary , the thickness of the elastic pad can be changed , and it may also be constructed like a mattress . the elastic pad may also be omitted . the bedstead 5 is provided with strength sufficient for bearing the weight of the bed plate and the patient . the medical bed 1 comprises a retaining device 9 for retaining the patient in a desired posture in such a manner that the patient is prevented from moving laterally along the bed plate 3 . specifically , a plurality of cylindrical recesses 11 , or holes , are formed on the bed plate 3 and the elastic pad 7 so as to perpendicularly pass through the pad 7 and extend inside the flat board 4 . a plurality of elongated members 21 are fitted to the recesses 11 so as to vertically stand on the bed plate 3 . as shown in fig2 and 3 , each elongated member 21 has a cylindrical pole 31 which has an upper blocking portion 31a and a lower fitting end portion 31b . the fitting portion 31b is removably and fittedly inserted into one of the recesses 11 of the bed plate 3 so as to firmly stand the pole 31 on the bed plate 3 . moreover , the blocking portion 31a of the pole 31 is covered with a cover 41 made of elastic material such as sponge and the like . according to the above construction , if the elongated members 21 are mated with the appropriately selected recesses 11 so as to bring the elongated members 21 in contact with the patient who is laid in a desired posture , each of the blocking portions 31a blocks motion of the patient and thereby prevents the patient from laterally moving along the bed plate . as a result , the patient cannot laterally move past the elongated members 21 and is thus retained in the desired posture . in the medical bed of the present invention , since the elongated members 21 can be fitted to specific positions among the many recesses 11 , the elongated members 21 can be adjustably positioned in accordance with the size of the body of the patient . in this construction , if the recesses 11 are too shallow , the elongated members 21 may be jarred loose by impact , for example . therefore , the recesses 11 preferably have sufficient depth for holding the fitting portion 31b . in the above construction , the elastic cover 41 serves as a shock absorber to reduce impact when the patient strains against the elongated members 21 . this cover can also serve to prevent the patient from slipping along one or more of the elongated members 21 . the following is an example of a method of retaining an infant patient on the above - described medical bed in a posture in which the patient is laid facing sideways for sampling cerebrospinal fluid of the patient . first , the patient is laid on the bed plate so as to lie sideways with knees and waist bent . next , one of the elongated members is fitted in one recess which is in the rear of the scapulae of the patient so as to abut aginst the back of the patient . then , another elongated member is fitted in another recess in front of the stomach of the patient so as to abut against the abdomen of the patient . finally , a third elongated member is fitted in a recess behind the knees of the patient . according to the above method of using the medical bed of the present invention , it is possible to quickly and efficiently retain the patient in the side - facing posture shown in fig1 . fig4 to 6 show modified embodiments of the retaining device according to the present invention . as shown in fig4 and 5 , it is possible to transversely extend the blocking portion of the elongate member to form a plate - like shape . in detail , the blocking portion 33a of the elongated member 23 of fig4 has a transverse section like that of a convex lens , and the blocking portion 35a of fig5 has a box shape . in these embodiments , each elongated member abuts the patient over a wide surface area , thereby dispersing any impact or forces exerted by the patient against the elongated member . fig4 also includes a modification of the fitting portion of the elongated member . in this embodiment , the fitting means commprises a rubber suction pad 33b coupled to the blocking portion 33a , instead of a cylindrical fitting portion . in this case , since the suction pad 33b enables the elongated member 23 to be adhered to a smooth surface , the elongated member 23 with the suction pad 33b is applicable only for a bed plate having a smooth surface with no recesses . fig5 and 6 show other modifications of the fitting portion of the elongated member . in each of these embodiments , a side surface of the fitting portion and the corresponding side surface of the recess on the bed plate are angulated to form a regular polygonal section , respectively . specifically , the fitting portion 35b and the recesses 15 of fig5 have a squared transverse section , respectively , and the fitting portion 37b and the corresponding recess 17 of fig6 have a regular hexagonal section . the above regular polygonal construction makes it possible to control the rotational position of the elongated member with respect to the longitudinal axis of the elongated member fitted to the bed plate . in fig5 the rotational position of the elongated member can be changed by 90 degree increment ( 360 / 4 degrees ), and that in fig6 can be changed by 60 degree increments ( 360 / 6 degrees ). in addition , fig6 shows an embodiment which includes an additional part , that is , a bent portion 37c . this portion can be used for preventing the patient from moving upwards relative to the bed plate . fig7 and 8 are plan views showing example arrangements of regularly disposed recesses in the bed plates . in fig7 the spacing of the recesses 11 along the longitudinal direction of the rectangular bed plate 3 is the same as those along the transverse direction . in fig8 the recesses 19 are provided in a regular hexagonal arrangement with the distance r being the same for adjacent recesses . of the above two arrangements , the arrangement of fig8 is more advantageous in regard to balanced strength of the bed plate . furthermore , in fig7 the recesses are formed on only half of the surface of the bed plate . this allows the other half of the bed plate to be used for general purposes not requiring the retaining of a patient . on the other hand , if the fitting portions of the elongated members are thinned within the limitation of maintaining durability , the size of the recesses can be reduced as well , so that the recesses don &# 39 ; t hinder the patient from lying comfortably . therefore , the recesses can be disposed over the entire surface of the bed plate , as shown in fig8 . in the present invention , only a few elongated members are required for retaining a patient , and this number is much smaller than the number of recesses formed in the above - described arrangement . therefore , even when elongated members are not being used , the area on the bed plate can be efficiently utilized by simply placing the elongated members in recesses along one side or near one corner of the bed plate . in the present invention , it is possible to construct the recesses as straight or curved grooves , instead of the cylindrical holes described above , in such a manner that the elongated members can slide therealong . moreover , it is also possible to arrange the medical bed so that the bed plate can be inclined relative to the bedstead at a desired angle . in addition , the bedstead may , of course , include a head frame , a foot frame and other accessories . in the above embodiments , the bed and the retaining device is used for medical treatment . however , it is , of course , possible to utilize the present invention at home for nursing a baby , caring for elderly people , and the like . it must be understood that the invention is in no way limited to the above embodiments and that many changes may be brought about therein without departing from the scope of the invention as defined by the appended claims .