Structure of Limulus Heart Muscle. 407 



The structure thus far described has been made out by teasing 

 in macerating fluids. For more definite work stained preparations 

 are necessary. The chief interest centers in cross and longi sec- 

 tions of the trabecular. Such sections determine the accuracy of the 

 ideas already obtained and also give the structure of the fibers making 

 up the trabeculse. 



When a specimen is sectioned parallel with the long axis of the 

 heart most of the trabeculse are seen in cross section. The field is 

 filled with oval, circular, triangular, or short ribbon-like areas. 

 These are the trabeculse^ and it is apparent that they vary greatly in 

 size and shape. The smallest are al)out 12 micra, and tlie largest 

 observed were al)()ut 90 micra in diameter. Near the basement 

 meml)rane the areas are closely appressed, while toward the lumen 

 there is considerable ^pace lietween. The arrangement is irregular 

 and the whole appearance is what one might expect from cutting 

 across strands that form a spongy heart wall. 



Figs. 5, 6, and 7 show four of these trabeculse in cross section. 

 The greater portion of each area is mostly filled with the cross sections 

 of the contractile fibrils which appear as black dots. These have 

 no very definite arrangement. In some cases they seem to have a 

 reticulated appearance, as in Fig. 7. Tn others they are in more solid 

 masses, as in Figs. 5 and 6. Only in the smallest is the strand ever 

 completely filled with fibrils. Between the fibrils sarcoplasm is of 

 course present. Clear areas are nearly always present in the cross 

 sections. Some of these may be due to vacuolization produced by 

 the fixative, but a certain number probably represent the poorly stain- 

 ing protoplasmic columns of the muscle fibers. In these columns 

 are found the nuclei of the muscle fibers. Surrounding the fibrillar 

 area is a thin cylinder of protoplasm which is bounded externally 

 by a fine but perfectly definite line. Within this boundary and 

 lying in the protoplasmic cylinder are found nuclei which in cross 

 section resemble those found in the protoplasmic cylinder. (See 

 Fig, 7 for illustration of these details.) 



The number of nuclei in the clear areas of the fibrillar portions 

 is variable ; there may be of course none, and there may be as many 

 as five or six in the larger trabeculre. When more than one nucleus 



