BLOOD-VASCULAR SYSTEM OF THE LORICATI 43 
tions, but for the most part they can be grouped under the head 
of Jongitudinal muscle fibers (Text-fig. 1, L.M.) and ¢ransverse 
muscle fibers (Text-fig. 2, T.M.). At various places the muscle 
fibers penetrate the connective tissue layer and very often fibers 
from the auricle would pass entirely through this layer into the 
ventricle. The auriculo-ventricular valves are merely a por- 
tion of the combined outer connective tissue layer, having their 
inner margins free, but having their outer edges securely bound 
down by muscle fibers. About the only difference in structure 
between the auricle and the ventricle, aside from the relative 
sizes of their central cavities, is the difference in the density of 
their muscle fibers. In the ventricle the blood cavities are very 
small, while in the auricle they are of considerable size, but 
growing smaller peripherally, and in the ventricle these cavities 
give place entirely to muscle fibers, peripherally. Cephalad in 
the ventricle the connective tissue gradually increases about 
the central cavity, marking the beginning of the conus arterio- 
sus. In 2 places folds of connective tissue pass entirely through 
the central cavity or conus as it is at this place. They are the 
semi-lunar valves (Text-fig. 2, S.V.). In places can be seen 
traces of elastzc muscle fibers (Text-fig. 2, E.M.F.) and round 
endothelium cells (Text-fig. 2, End.). While it is not the pur- 
pose of this paper to demonstrate the presence or absence of a 
layer of endothelium lining the central cavity of the heart, it 
may be said, however, that my sections did not show anything 
that I could positively identify as endothelium, until the origin 
of the conus was reached; nevertheless a silver impregnation 
would have doubtless revealed its presence. The walls of the 
bulbus are formed from 3 coats. ‘The external coat, tunica ex- 
terna or adventitza presents no peculiarities ; it is composed of 
longitudinal bundles of connective tissue, in which run the main 
nutrient vessels. Next comes the ¢uzz¢ca medza or middle coat, 
which is very thick and forms the longitudinal folds shown in 
fig. 40, which decrease in height as you go toward the ventral 
aorta. This tunic is constructed out of circular muscle fibers, 
in which run many white elastic fibers. The internal coat or 
tunica interna, which is made up of a longitudinal network, is 
bounded internally by a layer of large round endothelial cells. 
