67 
these fascicles were found on the gullet or termination of the pha- 
rynx opposite to the posterior border of the gill-cover. In Fishes 
the striated muscular fasciculi of the gullet appear, from the mea- 
surements now given, to be much smaller than the fasciculi of the 
muscles of the body ; and a like difference, though to a much smaller 
degree, often exists in mammals. 
In the heart of the smaller species of the lower Vertebrata di- 
stinct muscular fibres are often not to be found, the structure being 
less distinct than in the heart of many mammals ; generally composed 
of bands or fillets not easily separable from each other, and com- 
monly about = ,5th of an inch broad. ‘These fillets are seldom 
clearly streaked transversely ; they are irregularly and most minutely 
granulated, without the longitudinal arrangement of the granules 
so plainly visible in the beaded primitive fibrils of the heart of 
Mammalia. In short, the known points of resemblance between 
the muscular fibre of the heart of mammals and that of voluntary 
muscle are generally wanting in the structure of the heart of the 
smaller species of the lower Vertebrata, for the latter is more nearly 
allied to the muscular tissue of organic life as it exists in other 
parts. 
In some of the voluntary muscles of many of the smaller Mam- 
malia and Birds, as the common mouse and Fringillide, the existence 
of a sheath around the fibres appears to be questionable ; and in the 
heart of such animals the fibres are remarkably indistinct. In the 
common water-vole I noticed a very clear appearance of primitive 
fibrils, yet these seemed to be nowhere collected into fascicles. In 
the great pectoral muscle of various small birds, as the common swift, 
the transverse streaks are very indistinct, and often difficult to be 
seen, although they are very plain in the muscles of the leg; yet in 
this bird the former muscle is highly developed, and almost con- 
stantly in action, while the latter are but small and little used. It 
will be recollected that the above remarks apply only to particular 
muscles, and are not to be considered as at all invalidating the ad- 
mirable demonstration of the sarcolemma in many muscles by Prof. 
Schwann and Mr. Bowman, and the parallel observations of Dr. Jones 
Quain and Mr. W. J. E. Wilson*. As before observed, the fibres 
of the heart of Mammalia seem to have no intervening cellular (fila- 
mentous) tissue ; this tissue, however, is easily observed in the heart 
of many lower vertebrate animals; and I have very recently seen 
minute wavy filaments, having all the characters of cellular tissue, 
in the heart of the bear, and of some other mammals which had died 
in confinement. 
It will be seen that the term fibre, as used in this paper, corre- 
sponds to the primitive fasciculus of Fontana, Miller, and Bowmant. 
As in the heart there is often a tolerably clear appearance of fascicles, 
and as frequently only of the fillet-like bands, both have been set 
down in the following table as fibres: and the larger size of the 
fascicles will at once distinguish them from the bands, the primitive 
* See Phil. Trans. part ii. 1840, p. 475. t+ Loc. cit. p. 458, 
