On the Structure of Voluntary Muscular Fibre. 109 



In variety /3. the stem is more muricate. The leaves smaller, 

 and besides being inciso- or laciuiato-dentate, they are attenuated 

 more gradually into a longer and more slender stalk. The recep- 

 tacles are smaller, but present no other perceptible difference. 



In the absence of more perfect specimens, and indeed of a 

 larger series, the present description must necessarily be imper- 

 fect. The plant I have considered as a doubtful variety bears a 

 great resemblance to the other, yet 1 might perhaps with some 

 reason have raised it to the rank of a species ; the striking simi- 

 larity of the fructification alone deterred me. Should it prove 

 distinct, it may bear the name of S. pergracile. 



EXPLANATION OF PLATE IV. 



Sargassum j)orosum. 



Fig. 1. Leaves and vesicles on the young plant. 



— 2. One of the lower leaves. 



— 3. Leaves and vesicles on the fertile branches. 



— 4. Leaves of the ramuli with receptacle. 



— 5. Portion of a branch with old racemes, after the leaves and vesicles 



have disappeared. The two last magnified. 



Sargassum elegans. 

 Fig. 1. A branch. 



— 2. Leaf from ditto. 



— 3. Raceme. 



— 4 & 5. Raceme. 



— 6. Vesicles. 2, 4, 5 and 6 magnified. 



Sargassum brevifolium. 



Fig. 1. Lower portion of a branch. 



— 2. Raceme of fructification, with vesicles. 



— 3. Raceme, vesicles and leaf. 



— 4. Vesicle. 



— 5. Portion of var. /3. 



— 6. Leaves of ditto. 3, 4 and 6 magnified. 



XII. — Observations on the Minute Structure and Mode of Con- 

 traction of Voluntary Muscular Fibre ; being the abstract of a 

 Paper read before the Royal Medical Society, Edinburgh, De- 

 cember loth, 1848. By W. Murray Dobte, F.B.S.E. 



[With a Plate.] 



The structure of cross-striated muscle is a subject which has 

 more or less engaged the attention of minute anatomists, since 

 the first introduction of the microscope as a means of histological 

 research. 



There is perhaps no animal texture as to the nature of which 

 more contrary opinions have been held, or more conflicting state- 



