NERVOUS CONNECTIONS OF THE MAMMALIAN SPLEEN, INCLUDING AN ACCOUNT OF CERTAIN VISCEROMOTOR AND OTHER ABDOMINAL REFLEXES

Abstract

1. The splenic nerve of the dog contains at least two sets of efferent fibres, distinct in their anatomical arrangement and in the form of contraction they induce, the one set being probably arteriomotor, the other trabeculomotor.2. Afferent fibres are present in all trunks of the splenic nerve.3. Certain discrete segments of the splenic vein and of its main branches are endowed with special contractility. The segments in question form the terminal stations of particular nerve‐twigs from a main venomotor trunk, electrical stimulation of which thus causes local bands of contraction in the specially innervated regions.4. The splenic venomotor fibres join the left phrenic nerve in the mid‐cervical region. Coursing with it as non‐medullated fibres, they eventually perforate the diaphragm, where for a time they accompany the inferior phrenic artery. Deviating towards the cceliac ganglion, they next join company with the splenic vein, and are eventually distributed to localised parts of the vein.5. Physiological dilatation of the spleen is due to passage of nervous impulses down these venomotor fibres. This conclusion follows from the fact that on electrical stimulation of these fibres the resulting engorgement of the spleen corresponds in degree and in duration to the contraction of the splenic vein; from the further fact that when the venomotor fibres degenerate after section, the spleen remains in a state of partial contraction; and from the fact that section of these fibres abolishes reflex dilatation. The venomotor fibres thus form the efferent pathway for physiological dilatation.6. The muscles of the ventral abdominal wall are maintained in a condition of steady postural tone, which, however, is momentarily inhibited at the commencement of each inspiration.7. When any given abdominal muscle is suddenly stretched or squeezed, both it and the other abdominal muscles on the same side exhibit a strong reflex contraction. At the same time the tone of the abdominal muscles of the opposite side is by reciprocal action temporarily inhibited.8. Increased tonic contraction, unaccompanied by inspiratory relaxation, occurs in particular regions of the ventral abdominal musculature as a result of effective stimulation of particular viscera through their afferent nerve fibres. The portion of this musculature innervated from the 9th thoracic segment upwards on the left side is particularly responsive to stimulation of the afferent nerve fibres of the splenic nerve. Under more powerful stimulation of the splenic afferents, the region involved in reflex contraction may extend from this special splenic area to a correspondingly delimited area on the opposite side. The above observations may well have a corresponding clinical application.9. The afferent fibres for the splenic visceromotor reflex enter the spinal cord by the 5th, 6th, 7th, and 8th dorsal roots and are confined to the left side.