Resides being excited to action by impulses coming from the brain - volition - and from the surface - reflexion - the groups of cells in the spinal cord may act without any obvious incoming impulse; that is to say, some of the cells appear to be capable of independent activity. Such groups of nerve cells are commonly called automatic centres; the more important of those found in mammalia may be classified as follows:
1. Vasomotor centres: Though the central point controlling the contraction of the blood vessels is situated in the medulla, there is no doubt that even in man, centres are distributed throughout the gray matter of the spinal marrow, which are capable of keeping up the arterial tone in the regions over which they preside. As evidence of this may be mentioned the fact that the dilatation of the arteries, which follows the severance of the lumbar part of the cord from the medulla, only lasts a few days, after which the vessels again contract in a distinctly tonic manner. The arterial tonus only disappears completely and permanently when the spinal cord is destroyed. Thus, it would appear - although habitually all the vessels of the body are regulated by a centre in the medulla, nearly related to the cardiac centre - that every vascular region has a nervous mechanism of its own in the cord, which suffices to keep up the tonic contraction of the muscular coat of its vessels.
2. Sweating centres: Though closely related to the preceding, the centres which preside over the secretion of sweat in the lower part of the body and hinder extremities must, for many reasons which cannot now be mentioned, be regarded as separate centres.
3. Some smooth muscle fibres appear to be influenced by centres in the cord. In the lower part of the cervical cord is a group of nerve cells which keep the sphincter muscle of the iris in check; narrowing of the pupil has been described as following injury of this region.
4. The gray matter of the cord is also said to keep the skeletal muscles in a state of slight tonic contraction; elongation of the muscles is said to follow section of the anterior roots. When this muscular tone is absent the phenomenon known as "tendon reflex " is wanting, as the tap on the tendon ceases to excite the toneless muscle.
5. So-called trophic centres are also said to exist in the spinal cord. The best evidence in this matter is derived from the skeletal muscles. If the motor nerves or roots be cut, or the anterior gray motor columns injured, the paralyzed muscles soon undergo fatty degeneration, which does not depend on mere inactivity, for it does not follow cerebral paralysis, in which the integrity of the muscle can be preserved by suitable electric stimulation. Similar trophic agencies probably influence the other tissues. Thus, many affections of the skin, herpes, etc., are attributed to nervous lesions.
On account of the elaborate and purposeful reflex movements performed by decapitated frogs or eels, it has been suggested that in the lower vertebrates the spinal cord is capable of sensation and volition - mental activity - but to follow this assumption we should have to modify our ideas of volition and sensation, for which consciousness is commonly taken to be a necessary factor. It is, however, important to note that the lower we go in the scale of vertebrate animals the less powerful are the mental faculties, and the more important are the functions presided over by the spinal marrow.