This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The muscles hitherto spoken of piece of muscular ore highly magnified. At a the fibre has been crashed and twisted so as to tear its contents, while the tougher sarcolemma. elsewhere so closely applied to the rest as to be invisible, remains untorn and conspicuous are all more or less under the control of the will ; we can make them contract or prevent this as we choose; they are therefore often called the voluntary muscles.* There are in the body other muscles whose contractions we cannot control, and which are hence called involuntary muscles; they are not attached to the skeleton directly, nor concerned in our ordinary movements, but lie in the walls of various hollow organs of the body, as the stomach (Fig. 34), the intestines, and the arteries; by their contractions they move things contained in those cavities. Like the voluntary muscles, the involuntary consist of contractile elements, with accessory connective tissue, blood-vessels, and nerves; but their fibres have a very different appearance under the microscope. They are not cross-striped, but are made up of elongated cells united by a small amount of cementing material. Each cell (Fig. 35), is flat-tish, and tapers off toward its ends; in its centre is a nucleus with one or two nucleoli. The cells have the power of shortening in the direction of their long axes.
Fig. 33. A email.
Of what is a tendon made ?
Of what is striped muscular tissue composed ? Describe the form and size of muscular fibres?
What is the sarcolemma? What is the consistency of the contractile part of a living muscular fibre? What appearance does it present under the microscope? What is the cause of death stiffening? What are fibrilae ?
What do we mean by voluntary muscles?
Fig. 34. The muscular coat of the stomach.
What by involuntary? Which kind is attached to the skeleton? Where do we find the involuntary muscles?
* No sharp line can be drawn between voluntary and involuntary muscles; the muscles of respiration are to a certain extent under the control of the will; any one can draw a long breath when he chooses. But in ordinary quiet breathing we are quite unconscious of their working, and even when we pay heed to it our control of them is limited ; no one can hold his breath long enough to suffocate himself. Indeed, any one of the striped muscles may be thrown into activity, independently of or even against the will, as we see in the " fidgets" of nervousness, and the irrepressible trembling of extreme terror. Functionally, when we call any muscle voluntary, we mean that it may be controlled by the will, but not that it necessarily always is so. Structurally, the heart occupies an intermediate place; its striped fibres resemble much more those of voluntary than of involuntary muscles, but its beat is not at all subject to the will; though, as the exception proving the rule, it may be noted that there is an apparently well-authenticated case of a person who could by an act of will stop his heart.
Fig. 35. Unstriped muscle-cells.