No. I.] STUDIES ON LIMULUS. I 29 



where a few muscle fibers (w^.j.) are inserted upon the invagi- 

 nated lips and attached to the neural wall of the heart. Upon 

 the outside of the heart a grating (^r.) of elastic fibers stretches 

 across the ostia and prevents the lips from separating too 

 much. The valves are thus kept from being evaginated by the 

 pressure of the blood when the heart contracts. 



Histology of the Heart. — A cross-section of the heart 

 shows the cardiac walls to be composed of three layers: (i) 

 a median, dense, connective-tissue, basement membrane (PI. IX, 

 Fig. 7, b.mem.) ; (2) an outer longitudinal layer of thick elastic 

 fibers {I.C.S.); and (3) an inner muscular layer {a.m./.). The 

 muscle fibers, which are distinctly cross-striated, are inserted 

 upon the basement membrane and extend across the lumen of 

 the heart, branching, and anastomosing with each other. They 

 are loosely arranged around a central lumen, and form a circu- 

 lar layer thickest in the lateral angles of the heart. There is 

 no endothelium, and the blood circulates freely around the indi- 

 vidual muscle fibers. This muscular layer extends from the 

 extreme posterior limit of the heart to a point on the neural 

 side, opposite the aortic valve. On the haemal side (PI. IX, 

 Fig 5) its anterior boundary curves around the posterior end of 

 the aortic valve. 



In each angle of the heart is a longitudinal nerve (Text- 

 figs. 2-4; Pis. VIII-X, Figs. 3, 5-10, m.c.n. and l.c.n.) lying 

 between the elastic fibers, outside of the basement membrane, 

 but enclosed by a sheath which is continuous with the mem- 

 brane. 



b. Arterial System. 



The origins of the arteries from the heart in the normal con- 

 dition have been described above. There is, however, often 

 considerable variation. The arteria frontalis (Pis. VIII and 

 IX, Figs, 3-5 and 8, f.a.r.) may arise directly between the 

 two aortic arches, or from either one, or it may be absent 

 entirely. When present it gives off a small branch, which 

 goes posteriorly, haemal to the heart, and supplies the inter- 

 tergal muscles, areolar tissue, and epidermis in the haemal 

 median line. The main trunk runs forward in the median 



