STRIATED MUSCLE. 85 



which terminate in contact with the cardiac muscle fibers. Lymphatic 

 vessels are found in the larger layers and bands of connective tissue, but 

 they end before penetrating between the separate fibers. 



Although the cardiac muscle fibers form a network, they are in layers, 

 each having one general direction. Since the predominant direction varies 

 in different parts of a single section it is possible to find places where 

 the fibers are mostly cut lengthwise as in Fig. 94, and others where they 

 are cut across (Fig. 100). Here transverse bands and intercalated discs 

 cannot be seen. The nuclei surrounded by some protoplasm are near 

 the centers of the fibers. The fibrils cut across appear as dots which shift 

 about but do not disappear on focusing, since even in thin sections they 

 are not granules but short perpendicular rods. They are arranged in 

 radiating lines, or in clumps known as muscle columns. Close to the inner 

 lining of the heart the muscle fibers may be imperfectly developed, con- 

 taining only a peripheral ring of fibrils. These fibers (of Purkinje) are 

 abundant in the sheep but are infrequent in man. 



COMPARISON OF MESENCHYMAL MUSCLES. 



Smooth muscles are slender mesenchymal cells containing contractile 

 fibrils which are not banded. The cells, surrounded by a fibro-elastic 

 network, are generally closely associated in layers. If the border fibrils 

 actually pass from cell to cell, as has been said, then smooth muscle, like 

 other muscle, is syncytial in nature. 



Cardiac muscle is a syncytium of mesenchymal origin, consisting of 

 broad approximately parallel branches. It contains banded contractile 

 fibrils not limited by cell areas. It is distinguished from smooth muscle 

 by its cross striations and by the width of its fibers; and from striated 

 (voluntary) muscle by its mesenchymal origin, the branching of its syncy- 

 tium, the central position of its nuclei, and the possession of intercalated 

 discs. 



STRIATED MUSCLE. 



Striated muscle, as the term is ordinarily used, does not include the 

 striated cardiac muscle, but only the striated muscle which develops from 

 the epithelium of the mesodermic segments [proto vertebrae]. The segments 

 form a series of paired masses of cells found on either side of the medullary 

 tube. They have been briefly described on page 22. At first they are 

 epithelial structures bounding a part of the coelom or body cavity. Later 

 they lose their connection with the coelom (Fig. 21) and become rounded 

 masses of cells, each mass enclosing a cavity. From the median side 

 of the segment, near its ventral border, a stream of mesenchymal cells is 



