ments at each extremity, placed end to end. The spaces between the contracting elements are filled by a clear substance, called sarcoplasm. As the sarcostyles run parallel with their composing elements, opposite each other, the appearance of the contiguous thick centers Iwill be that of the dark band. The thin ends, surrounded by clear sarcoplasm, will show as the light band. The enlargements at the thin ends will produce a row of granules, which are so closely placed as to appear as a line the membrane of Krause. The median line of Hensen is produced by a slight thinning of the contracting elements just at their centers, and by refraction of light. The individual fibers of voluntary muscle are bound together by very delicate fibers of white fibrous tissues, called the endomysium, to form bundles (the fasciculi). These fasciculi are surrounded and bound to other fasciculi by a larger band of white fibrous tissue, called the perimysium, and these fasciculi are grouped together to form the large muscles as found in dissection, and are surrounded by a still larger mass of fibrous tissue (the epimysium), which constitutes the sheath of the muscle. These connective tissues serve also to support numerous nerves, bloodvessels, and lymphatics. The blood-vessels break up into capillaries, which form a fine network around the individual muscle-fibers. The nerves terminate in special end-organs under the sarcolemma.

SMOOTH MUSCLE.

Smooth, plain, involuntary, or nonstriated muscle is found chiefly in the gastro-intestinal tract, the arteries around the various ducts, in the genitourinary tract, and in the skin and capsules of organs. It contracts slowly and is not subject to the will. Smooth

SMOOTH MUSCLE.

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muscle is composed of spindle-shaped cells, covered by a fine sheath, which, while varying considerably in size in various locations, may be said to average about of an inch in length and of an inch in width. In the center of each fiber is an elongated, rod-shaped

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nucleus. Very fine longitudinal striations may be seen in especially favorable sections. The fibers are held together by cement substance and by interlacing of their ends. Occasionally, extremely fine spicule branch at right angles to the fibers and aid in uniting them.

HEART MUSCLE.

Heart or cardiac muscle, as its name implies, is found only in the heart. It is composed of rectangular, branching cells, placed end to end, and joined by a small amount of clear cement substance. The fibers are more delicately cross-striated than the voluntary muscle, and, like the latter, possess fine longitudinal markings.

A single oval nucleus is present in the center of each cell. Small amounts of fibrous tissue and capillaries are found between the cells.

The following tabulated comparison of the three types of muscle may be of value in distinguishing them :

QUESTIONS FOR CHAPTER III.

50. Name the varieties of muscle.

51. Give the size of the voluntary muscle-fiber.

52. What is the sarcolemma?

53. What is the membrane of Krause?

54. What is the median line of Hensen?

55. What is the sarcous element?

56. Describe the structure of a voluntary muscle-fiber.

57. What kind of tissue binds together the voluntary musclefibers?

58. What is this fibrous tissue called in various locations? 59. Describe the blood supply of voluntary muscle.

60. How and where do the nerves terminate?

61. Describe the nuclei of voluntary muscle.

62. What is the shape and size of smooth muscle-fibers?

63. Where is smooth muscle found?

64. Describe the structure of smooth muscle.

65. How are the smooth muscle-fibers held together?

66. What is the shape of cardiac muscle?

67. Describe its structure.

68. How are the fibers held together?

69. What are the differences between voluntary and smooth muscle? voluntary and heart? heart and smooth?