615 
At the periphery of the cell the fibril bundles are surrounded in 
the same way by small sarcoplasmic discs, so that their outer limits 
make up the boundaries of the cell, forming a sort of cell membrane. 
The sarcoplasm surrounding the nucleus at the centre of the cell, 
where fibril bundles are absent, is made up of similar discs, but here 
they are somewhat larger than at the periphery. 
The main features to be observed in a cross section then are 
the following: 
1) The presence of fibril bundles usually in the form of leaf-like 
bands, arranged radially around the periphery of the cell, internal to 
which are smaller polygonal masses, situated irregularly between the 
peripheral radially disposed masses and the central perinuclear sarco- 
plasm. 
2) The breaking up of the sarcoplasm into regular small discs 
which surround the fibril bundles. The membranes of those small 
discs which reach the surface of the cell go to make up the external 
limits of the latter. 
It is to be emphasized F 
then, that in the protoplasm 
of the adult heart muscle cell, 
there are columns of fibrils 
which run longitudinally sur- 
rounded by sarcoplasm, in the 
way that each bundle is sur- 
rounded by a varying number 
of small sarcoplasmic discs, the 
horizontal separating partitions 
of which are continuous with 
Krause’s line on the fibril bun- 
dles. This may be schema- 
Fig. 5. Diagram of the structure of adult 
tically represented as in Fig. 5. human heart muscle. 7 fibril bundle; S small 
sarcoplasmic dise; @ Querscheibe, 
Heart Muscle of other Animals. 
The muscle found in hearts from other animals is essentially the 
same in structure as that described above, differing only in the mode 
of union of the cells. The heart tissue of the pig, dog, cat, rabbit 
and mouse was examined, and the structure in every case was found 
to be practically identical with that described for human heart muscle. 
In the heart of the sparrow there are spindle-shaped cells, con- 
taining fibril bundles surrounded by small sarcoplasmic dises, as de- 
scribed above. 
