So far reference has only been made to the skeletal muscles, the fibres of which are marked by transverse striations, and whose single contraction is extremely rapid and short. The contractile tissues which carry on the movements in the various organs of the body are not striated fibres, but, as has been already stated, consist of elongated flattened cells with rod-shaped nuclei. They occur generally in the form of sheets or layers, forming coats for the organs in which they lie. Their single contraction is slow and prolonged, and is generally transmitted from one muscle cell to another as a kind of sluggish wave. They are not capable of passing into a tetanic state of contraction, like striated muscles.
The slowest contraction seems to be that of the muscle cells in the walls of the blood vessels. These remain in a state of partial contraction, which undergoes a brief and temporary rhythmical relaxation. The most forcible aggregate of unstriated muscle elements is met with in the uterus. This organ, which has very exceptional motor powers to perform, contracts in somewhat the same way as the muscles of the blood vessels, but more quickly, and with longer rhythmical intervals of partial relaxation. The muscular wall of the intestine, and the iris, are among the most rapidly contracting smooth muscles.
The chemical properties of the smooth muscle are somewhat similar to those of striated skeletal muscles, and they pass into a state of rigor, while dying, which seems to depend on the same causes as the rigor mortis already described.