[18] Wertheimer: Archives de Physiologie, 1892, xxiv, p. 379.
[19] Pawlow: Loc. cit., p. 56.
[20] Mantegazza: Fisiologia del Dolore, Florence, 1880, p. 123.
CHAPTER II
THE GENERAL ORGANIZATION OF THE VISCERAL NERVES CONCERNED IN EMOTIONS
The structures of the alimentary canal which are brought into activity during the satisfactions of appetite or are checked in their activity during pain and emotional excitement are either the secreting digestive glands or the smooth muscle which surrounds the canal. Both the gland cells and the smooth-muscle cells differ from other cells which are subject to nervous influence—those of striated, or skeletal, muscle—in not being directly under voluntary control and in being slower in their response. The muscle connected with the skeleton responds to stimulation within two or three thousandths of a second; the delay with gland cells and with smooth muscle is more likely to be measured in seconds than in fractions of a second.
The Outlying Neurones
The skeletal muscles receive their nerve supply direct from the central nervous system, i. e., the nerve fibres distributed to these muscles are parts of neurones whose cell bodies lie within the brain or spinal cord. The glands and smooth muscles of the viscera, on the contrary, are, so far as is now known, never innervated directly from the central nervous system.[*] The neurones reaching out from the brain or spinal cord never come into immediate relation with the gland or smooth-muscle cells; there are always interposed between the cerebrospinal neurones and the viscera extra neurones whose bodies and processes lie wholly outside the central nervous system. They are represented in dotted lines in [Fig. 1]. I have suggested that possibly these outlying neurones act as “transformers,” modifying the impulses received from the central source (impulses suited to call forth the quick responses of skeletal muscle), and adapting these impulses to the peculiar, more slowly-acting tissues, the secreting cells and visceral muscle, to which they are distributed.[1]
[*]The special case of the adrenal glands will be considered later.
The outlying neurones typically have their cell bodies grouped in ganglia (G’s, [Fig. 1]) which, in the trunk region, lie along either side of the spinal cord and in the head region and in the pelvic part of the abdominal cavity are disposed near the organs which the neurones supply. In some instances these neurones lie wholly within the structure which they innervate (see e. g., the heart and the stomach, [Fig. 1]). In other instances the fibres passing out from the ganglia—the so-called “postganglionic fibres”—may traverse long distances before reaching their destination. The innervation of blood vessels in the foot by neurones whose cell bodies are in the lower trunk region is an example of this extensive distribution of the fibres.