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Below the second lumbar segments the spinal cord shows certain peculiarities, the result, evidently, of the processes of cephalization and of adaptation to the requirements of those organs not subject to the process of cephalization to any great extent. The progressive loss of the lower spinal segments, the tendency of the controlling centers to become superseded by the increasing efficiency of the cephalad nerve centers, the constant crowding of the visceral centers by the development of the leg-muscle centers, and the simultaneous rearrangement of the connections within the spinal gray matter, all add to the lumbo-sacral cord something of the complexity of relationship found in the medullary centers. In these segments of the cord the gray matter is arranged in the form of a very short, broad H. The anterior horns contain several cell groups arranged in columns. The mesial cell group includes at least two columns. The axons of these cells innervate the trunk muscles. The lateral cell group of the anterior horn is divided into two, three or four columns by different authors. The axons of these cells innervate the muscles of the lower limbs. The lateral horn is not recognizable in the second, third and fourth lumbar segments, but the viscero-motor functions seem to be performed by the cells lying in the lateral portion of the broad H. The fifth lumbar segment begins to show the lateral horn structure, and the sacral segments display the lateral horn cells as the origin of the nervus erigens. The posterior horns of the lumbo-sacral segments contain a number of small cells, such as are found in the posterior horns throughout the spinal cord. The dorsal nucleus (Clarke’s column) is wanting. The axons of the cells of the mesial group columns of the anterior horn innervate the following muscles: Multifidus spinae, interspinales, psoas major and minor, gemelli, glutei, obturators, pyriformis, and the lower parts of the abdominal muscles. The axons of the cells of the antero-lateral columns supply the muscles of the thigh, leg, foot and toes. The location of the cells which supply a number of the pelvic muscles is not exactly known. They seem in part to be homologous with somatic muscles and in part with muscles of visceral function. Since no white rami or homologous nerve fibers are found in the lumbar region below the first lumbar, or at most the second, and only the nervus erigens is found in the sacral segments, it is evident that the phylogeny of the muscles named below needs considerable study. The muscles supplied form the lumbo-sacral segments in this manner are: Cremasters, sphincters ani, levator ani, coccygeus, compressor urethrae, ischio-cavernosus, bulbo-cavernosus, transversus perinel, sphincters vesicae, and the sphincter vaginae. The centers controlling the muscles first mentioned, as well as those of the thigh, leg, foot and toes, receive impulses from the following sources: I. Descending impulses from the precentral convolution of the cerebral cortex affect the activities of all the muscles named, but chiefly those of the lower limbs. II. Descending impulses from the cerebellum, vestibular nuclei, olive, quadrigeminates, medullary centers and the centers of the reticular formation, maintain the normal tone of all the muscles innervated from these segments, and coordinate their action in such a way as to secure equilibrium and to provide for the delicacy of certain movements, notably those employed in walking, running, etc. III. Descending impulses from the red nucleus and associated centers bring these muscles under the control of the emotional states. IV. Somatic sensory impulses from the skin, muscles, joint surfaces, etc., innervated from the same segments of the cord, assist in the maintenance of the muscular tone, in the simple reflex actions, and in part in the control of the centers which are apparently of viscero-motor function. The somatic sensory impulses from joint surfaces abnormally related, abnormally contracted muscles and the like, may bring about abnormal contractions or the loss of the normal tone of the muscles innervated from the mesial cell groups, and of the muscles of the sphincters. V. Visceral sensory impulses may affect the activity of these centers. The abnormally contracted muscles found associated with diseases of the pelvic viscera are produced through the intermediation of these reflexes. VI. Descending impulses from the medullary visceral centers coordinate the activities of the lumbo-sacral centers with the activities of other nerve centers. VII.
Ascending and descending impulses from the lumbar and sacral centers concerned
in the coordination of certain complex acts affect the centers governing
the muscles named.
This center is concerned in the control of all the structures involved in defecation. The exact location of the nerve cells is not known, beyond that it lies in the lumbar enlargement. (Fig. 46.) Both stimulatory impulses to the muscular fibers of the lower bowel and inhibitory impulses to the anal descending tracts of association and the third and fourth sacral nerves or through the white rami of the first lumbar segments, the sympathetic ganglia and the inferior mesenteric and hypogastric plexuses. Impulses are sent upward also to thoracic centers for the stimulation of the abdominal and thoracic muscles. The ano-spinal center is controlled by impulses from the following sources: I. Sensory impulses are received from the region of the rectal and anal tissues, by means of which the normal impulses of defecation are aroused. II. Sensory impulses are received from neighboring tissues, by means of which defecation movements may be inhibited, or at times initiated in the absence of the normal stimulation. This relation is evident under abnormal conditions, as in vaginal abnormalities, hemorrhoids, and other irritations of the region. III. Sensory impulses, mostly inhibitory in effect, may reach the center from the somatic structures. This relation is seen in the effects of lesions of the peri-anal tissues, and especially in the effects of malpositions of the lumbar vertebrae. IV. Descending impulses from the cerebral cortex seem to act in a rather complex manner. First, the descending volitional impulses cause excessive contraction of the external muscles, including the glutei, etc. The sensory impulses from these muscles assist in the inhibition of the action of the ano-spinal center. The pressure exerted upon the descending column of feces by volitional contractions of the skeletal muscles probably lessens the sensory impulses reaching the center from the rectal membrane. Perhaps the descending inhibitory impulses act directly upon the ano-spinal center, though it is not yet possible to demonstrate this manner of control. Descending impulses from the cortical centers may assist in stimulating the ano-spinal center to a certain extent. This relation is used in the attempt to establish the “habit” control by directing the patient to go to stool at certain times of the day, and to attempt to perform the act of defecation by using the abdominal muscles, etc., as in the normal act. This attempt brings about the stimulation of the nerve cells which control the skeletal muscles. Since the relationship between these cells and those concerned in the autonomic centers is very intimate, the stimulation of the defecation center is probably secured. This attempt to secure normal defecation is logical only when efficient causes of the abnormal function have been or are being removed. As in the case of the descending inhibitory impulses, we must recognize the possibility of a direct stimulation of the cells of the ano-spinal center from the cerebrum, though this must not be held as demonstrated, and now appears extremely unlikely. V. Descending impulses from the cerebellum, quadrigeminates, olive, and the medullary and pontine centers, seem to be concerned in the maintenance of the normal tone of the muscles concerned in defecation, though these impulses are not necessary, since the act may occur in a normal manner, unconsciously, in men and animals in whom the cord has been severed or destroyed. VI. Descending impulses from the red nucleus and other basal ganglia may cause defecation to be affected by the emotional states. Thus defecation may be either inhibited or be performed involuntarily under emotional stress. VII.
The associations between the defecation, micturition, erection, and parturition
centers are very intimate. Thus defecation may be initiated involuntarily
or may be inhibited by the different states of these other centers.
Micturition is accomplished in much the same manner as defecation, and the vesico-spinal center is influenced by nerve impulses from the same sources. The
lack of control of the vesical sphincter in children and sometimes in adults
may be associated with slight malpositions of the lumbar vertebrae, the
sacrum, and the innominates. Certain peripheral irritations, as from
worms, or from adherent prepuce or hooded clitoris, may affect the vesico-spinal
center.
This center has been demonstrated for male dogs and for men, but no homologous center has been demonstrated in the female. Impulses from this center act as inhibitors of other lumbar centers and of many medullary centers. The efferent impulses pass to the various centers for the skeletal muscles concerned in coition and to the erection center. The activities of the genito-spinal center are influenced by impulses from the following sources: I. Sensory impulses from the genital tract, and to a certain extent from neighboring tissues. II. Descending impulses from the red nucleus and neighboring basal ganglia. These impulses are concerned in the emotional states, and may be either stimulatory or inhibitory. The ill effects of the reading of erotic literature by young people may be traced in part to the impulses aroused in the emotional centers by such reading. III.
Descending impulses from the cerebrum probably act indirectly through the
action of the volitional impulses upon other structures and through the
influence of the descending volitional impulses upon the basal ganglia.
This center lies in the lower part of the lumbar cord. It is to be considered a vaso-motor center, and it is in very close association with the genito-spinal center. Its efferent impulses pass downward for the most part to the origin of the nervi erigentes. This center is influenced with and by the genito-spinal center, but seems to be somewhat more easily affected by sensory impulses. In certain cases of malpositions of lumbar vertebrae, sacrum, coccyx and innominates the activity of the erection center may become abnormal. Whether the bony lesion is ever the only cause of these disturbances is not certainly known, but the correction of the lesions, together with the correction of whatever may be found wrong from the hygienic standpoint, is sometimes followed by very speedy and apparently permanent relief in the cases not associated with local structural causes of the disturbances or with disease of the spinal cord. This center lies in the upper part of the lumbar cord. It seems to be concerned more with the regulation of labor than with its initiation. The parturition center acts by sending impulses to the uterine muscle, to the dilator center for the cervix uteri, to the centers for the abdominal muscles, and to other centers in the lumbar cord. It is not certain whether the impulses affecting the action of the respiratory center are from the parturition center or from the other sources affecting the respiratory centers. The physiology of this center requires further study. The parturition center may be affected by impulses from the following sources: I. Sensory impulses from the uterine and neighboring tissues may decrease the uterine contractions, but usually increase them. Fig. 45. Simple visceral reflex arc. Gray fiber; Sympathetic cell; Anterior root’ Lateral horn; White fiber; Sensory; Cell. II. Descending impulses from the red nucleus and related ganglia may initiate, postpone or hasten labor. This is evident in the premature births associated with battles, disasters, and other emotional storms. It seems probable that labor may be postponed by excessive disgust or fear, and the emotional states may be responsible for other variations from the normal progress of parturition. III. Probably no direct volitional impulses affect the center, but volitional impulses may affect the abdominal muscles and the emotional centers, and thus may indirectly affect the progress of labor. IV. Somatic sensory impulses may perhaps be concerned in influencing the action of this center. It seems more probable that the place of slight malpositions of structural tissues in modifying labor is more prominent in affecting the nutritional and developmental conditions than as a factor in labor itself. This does not apply to the gross bony malpositions which mechanically affect the birth canal. V.
There is some evidence in favor of the view that the quality of the blood
flowing through the center may affect its activity.
These have not been well demonstrated, except as already given. The lumbar cord contains the vaso-motor centers for the uterus, the urinary bladder and the prostate. These centers seem to be affected abnormally by somatic sensory impulses from the structures innervated from the same spinal segments. Congestions of these organs are frequently found associated with malpositions of the lumbar vertebrae, the innominates, and the sacrum. Experimental stimulation of the somatic tissues innervated from the same segment of the cord is found to affect the circulation of these organs. Experimental stimulation of the visceral tissues innervated from the same segments does not produce such noteworthy effects. No explanation is offered for this phenomenon. The nervus erigens passes directly to the hypogastric plexus. It terminates by entering into the formation of the pericellular baskets of the small ganglia of this plexus. The gray fibers from these small ganglia pass to the blood vessels, chiefly of the external genitals. The visceral efferent impulses of the sacral centers pass chiefly by way of these paths. The circular fibers of the os uteri are inhibited by impulses from this center. The center seems to be affected chiefly from the parturition center in the lumbar cord. Experimental and clinical evidence seems to indicate that the activity of this center may be affected by sensory impulses from the somatic structures innervated from the same segments. Other relations of this center have not been sufficiently demonstrated. |
