BLOOD-VASCULAR SYSTEM OF THE LORICATI 43 



tions, but for the most part they can be grouped under the head 

 of longitudinal muscle fibers (Text-fig. i, L.M.) and transverse 

 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 elastic 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 adventitia presents no peculiarities ; it is composed of 

 longitudinal bundles of connective tissue, in which run the main 

 nutrient vessels. Next comes the tunica media 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. 



