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BY S. V. CLEVENGER, M.D.
[Continued.] T this stage the so-called afferent commissure alone is estab
lished, but the same law of unification of segments in the construction of an individual from its component colonial members will also confer upon it an efferent commissural system.
While this is intended to represent the visceral nervous system of Invertebrata, the same rule will apply in the union of vertebral ganglia segments in higher forms, beginning in such Invertebrata as possess more than one secondary system ganglia (some Arthropoda).
Ganglionary fusions occur in parasitic insects and other forms, but this is secondary and does not interfere with the general application. By omitting the afferent part of the fibers that form the commissures, the segmental union may be expressed thus, and confusion avoided :
These may be schematically expressed in diagrams which show the higher ganglionic series to be commissurally connected with the lower. Each higher segment presiding over a lower system series and the commissures between forming apparently, direct projection systems.
This scheme would explain why the splanchnics have no inhibi. tory control over the intestines (Ludwig and Haffter), such control really pertaining to higher projections (Ott).
Each spinal ganglion segment presiding over a similar series would be thus represented :
While each and every ganglion preserves its primary projection functions, the commissures interrelate the systems and are themselves projection fibers.
The third system in this diagram is incomplete in not being commissurally connected, hence it is but a ganglionic (intervertebral) swelling upon the secondary spinal afferent nerves, and attains its complete functional character within the cranium only.
In Trigla adriatica the brain and dorsum of the cord are marked by a double chain of well-developed tubercles on the secondary nerves just as they enter the cord. These ganglionic enlargements are exact reproductions of the cerebellar and cerebral enlargements, en chatelaine. These intervertebral ganglia constitute the third system, the difference between the spinal ganglia and those above being that the latter are commissurally united to one another, and in higher Vertebrata fused longitudinally.
The vermis of the cerebellum is probably a mere commissure compounded of different segmental heights, for the transverse striations are pronounced in birds and some reptiles.
According to Owen, the cerebellum (vermis) retains its embryonic form of a simple commissural bridge or fold in the parasitic suctorial Cyclostomes and sturgeon, and in the almost finless Lepidosiren, while the cerebellum (still vermis, because centrally placed) is highly developed in the sharks. In the saw fish it even rests upon the "cerebrum."
The first well-marked posterior ganglionary mass which in higher Vertebrata becomes a portion of the cerebellum, is the vagus tubercle, placed posterior to and below the "cerebellum ” of the fox shark. It might be safe to assume that subsequently this tubercle (third system ganglion) forms the flocculus or pneumogastric lobule of the cerebellum.
The Gasserian ganglion (unmistakably an intervertebral), where non-existent, must form a portion of the cerebellum.
The cerebellum then appears to be formed from fused hypertrophied intervertebral ganglia.
Stilling regarded the law of isolated conduction as inapplicable to the cerebellar lobes, owing to the very great commissural fused) union which occurs there. Thus'a coördinating function between cranial nerves on the one hand (the cerebellum acting as connected intervertebral ganglia for many cranial nerve fibers), and the general spinal system on the other, m'ist follow in such Vertebrata as are governed mainly by cerebellar supervision, while in higher forms it is brought directly into relation with the cerebrum itself.
Above this the cephalic intervertebral ganglia developed in some animals, atrophic or rudimentary in others, appears to be the posterior and anterior tubercula bigemina, epiphisis cerebri, eminentia mammillaria, olfactory lobes, cerebrum, which latter is itself composed of several lobes or ganglia, some of which, as the anterior, are undeveloped in most Vertebrata and even in many mammals.
The posterior bigeminal lobe appears to be a third system ganglion related to special tactile sense (see Spitzka, N. Y. Medical Record, March 13, 1880), while the optic lobes (anterior bigeminal) are third systems for the optic nerves. The primitive optic fibers were related to the gray matter above the chiasma, and even in man retain some primary thalamic connections.
The epiphisis cerebri (pineal gland), bilobed in the fætus and devoid of sabulous matter in forms below man, attains quite a large size in some animals (Meleagris gallapavo, p. 260 “Huxley's Vertebrates ”). It may with the mammillary eminence indicate a sense between sight and olfaction.
The mammillary eminences can be third systems, their positions and cinereal envelope weighing nothing against the idea, for the Teliost cerebrum itself drops to a comparably defective structure and inferior position.
These eminences are very large in monotremes, marsupials and the horse. They stand related to the fornix, which in turn is connected to the olfactory lobe.
The olfactory lobe (another third system ganglion) appears to have been derived from a place lying in front of the mammillary eminences, according to Luys' sections, but Meynert is doubtless more correct in attaching the olfactory primitively to the optic thalamus.
The olfactory lobes, of more importance in some vertebrates than the cerebrum, in man became strangled, so to speak, by the preponderance of higher third systems.
(“The olfactory lobe bore such important relations to the life history of early vertebrates that we are not surprised to find the cerebral hemispheres developing at first as mere appendages of the olfactory lobes."-Spitzka, “Architecture and Mechanism of the Brain," p. 37.)
The lobes of the cerebrum are related to the corpus striatum, which seems to be a part of the medullary gray second system, though formed after the hypophysis cerebri had become atrophic as the end of the spinal cord.
The hypophysis ended in the sella turcica and the corpus striatum (caudate nucleus), and subsequently lenticular nucleus developed in the scale of intelligence (Meynert).
In Teliost fishes the optic lobe (third system) occupies the place of the cerebrum of mammals in point of mass development, and the inference is natural that this optic lobe contains the highest centers related to the psychic life of this division of vertebrates. The cerebrum proper being an undeveloped tubercle in front of the mammillary eminence with the infundibulum between them (Todd, p. 619, Vol. 11).
In Amphioxus we have the culmination of the secondary ganglionic type with the foreshadowing, seemingly, of the tertiary, in the black pigmentary formation in the dorsal portion of the notochord. This vertebrate, so far from being anomalous, explains by its rudimentary organization what appears later in the Cyclostomi or above. Its second pair of nerves runs from the dorsal segmental nerves to the head-end ganglion, thence to the ventral segmental nerves, typifying the medulla oblongata control over lower centers, without the intervention of a cerebellar or any other third system.
The optic ganglion (secondary) of the crab ( Carcinus manas) topographically precedes the antennal, from which may be inferred that the posterior bigeminal (tertiary) is related, as Spitzka claims, to the special tactile (fifth pair) sense.
The slight development of the superior ganglia in Brachiopoda is correlated with higher sensory organs, and Gegenbaur, p. 310, notices that the nerves for the arms are probably given off from the ventral ganglia, a condition which I suspect is more common than usually thought to be the case, due to the want of differentiation between alimentary and locomotor parts, so far at least as central innervation is concerned. “In the Mollusca the visceral ganglia are not only of importance, as forming a part of the general nervous system, but they may also fuse with the cerebral ganglia, owing to the gradual shortening of their commissures. New and primitively peripherally placed parts are thereby added on to the central organs, and it becomes a matter of doubt whether or no these ganglia, which formerly belonged to the visceral nervous system should still be regarded as belonging to it.”—Gegenbaur, p. 344.
The development of the nervous system appears to have proceeded as follows:
SECONDARY. Respiratory-Special tactile locomotory, auditory, optic, or optic and next auditory.
Antennal special tactile from which auditory in some ; (olfactory not certain in Invertebrata, possibly in Cephalopoda. In Vertebrata originate highest secondary and tertiary).
The progression of faculties intermingle and a branch sense appears often to develop indifferently from one or other trunk, as while respiratory may give rise to the tactile for locomotion, and audition follows upon this, the antennal for gustatory purposes may originate the auditory, while locomotor tactile may be developed separately.
NERVOUS ORGANIZATION OF INVERTEBRATA. 1. Protozoa.- Not perceptibly differentiated. 2. Cælenterates.-Rudimentary primary. 3. Vermes.
Secondary appears and becomes highly 4. Echinodermata. I developed. Often fused with primary.
Secondary well developed. In Insecta 5. Arthropoda. the primary quantitatively developed.
Tertiary pronounced in bee.