VOL. XX.] PHILOSOPHICAL TRANSACTIONS. 251 



as they ascended from the os pubis, grew slender, being about the middle 

 but 4- inch broad; and they were each 2 inches long. These bones were fur- 

 nished with 4 pair of muscles : and another pair ran over them, to which they 

 performed the office of a trochlea, or pulley. The first pair of muscles (i. e. 

 which first came to be dissected, on the pronation of the animal, and from its 

 figure I call triangularis) arises fleshy from the whole length of the internal 

 side of these bones, and inserted their opposite tendons on each side of the 

 rima, or aperture of the marsupium. Under part of the muscles, lay another, 

 or a 2d pair, flat and thin, having their origin from the upper part of the in- 

 ternal side of the oss;i marsupialia, and inserting their opposite tendons a little 

 above the tendons of the former muscles : the tendency or direction of the 

 muscular fibres of this pair, in respect of the first, made a decussation. The 

 3d pair of muscles had their rise from the fore-part of the basis of these 

 bones, where they were joined to the os pubis ; and were afterwards inserted 

 into the linera aspera of the thigh-bone. The 4th pair arose from the external 

 side of these bones near the basis, and are inserted into the fore-part of the 

 thigh-bone near the middle. The last pair of muscles arises more immediately 

 from the marsupium or pouch itself; for spreading their muscular fibres all over 

 this bag, as they issue from it, by joining their fibres together, they more 

 remarkably form a solid muscle ; which passing on each side over the middle 

 of these bones, i. e. in the prone posture of dissecting the animal, they at 

 length were inserted into the spine of the os ileum. 



By considering the structure of these muscles, and what must be the effect 

 of their action or contraction ; one cannot but think the first 2 must serve 

 towards the dilatation or opening the marsupium or pouch : for these bones are 

 a fulciment or basis, their articulation not admitting of a contraction inwards, 

 or nearer to each other; wherefore, when the 1st and 2d pair of muscles act, 

 or contract, they must necessarily open or dilate the mouth of the marsupium 

 or pouch. The 3d and 4th pair may serve to extend these bones outwards ; 

 so that when this animal hangs by its tail, as it frequently does, the weight of 

 the foetus in this pouch by this means will not press so much on the internal 

 viscera. The 5th and last pair, as they may serve to dilate the capacity of the 

 pouch itself, so likewise may serve the better to suspend its weight, when the 

 animal is prono capite, and if it gravitates too much, they may retract it up- 

 wards, and this the easier, as passing over these bones like a pulley, their force 

 is more augmented. The antagonist to these muscles is the sphincter marsupii, 

 an oval series of strong, fleshy fibres, which serve to constringe and close the 

 orifice of the pouch ; which it does so perfectly, that one would think the skin 

 here not to be slit ; nor can the orifice be observed till it is dilated with the fingers. 



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