The Works of Francis Balfour 1-9
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol I. Separate Memoirs (1885)
IX. On the Spinal Nerves of Amphioxus
Duringa short visit to Naples in January last, I was enabled, through the kindness of Dr Dohrn, to make some observations on the spinal nerves of Amphioxus. These were commenced solely with the view of confirming the statements of Stieda on the anatomy of the spinal nerves, which, if correct, appeared to me to be of interest in connection with the observations I had made that, in Elasmobranchs, the anterior and posterior roots arise alternately and not in the same vertical plane. I have been led to conclusions on many points entirely opposed to those of Stieda, but, before recording these, I shall proceed briefly to state his results, and to examine how far they have been corroborated by subsequent observers.
Stieda 2 , from an examination of sections and isolated spinal cords, has been led to the conclusion that, in Amphioxus, the nerves of the opposite sides arise alternately, except in the most anterior part of the body, where they arise opposite each other. He also states . that the nerves of the same side issue alternately from the dorsal and ventral corners- of the spinal cord. He regards two of these roots (dorsal and ventral) on the same side as together equivalent to a single spinal nerve of higher vertebrates formed by the coalescence of a dorsal and ventral root.
Langerhans 3 apparently agrees with Stieda as to the facts about the alternation of dorsal and ventral roots, but differs from him as to the conclusions to be drawn from those facts. He does hot, for two reasons, believe that two nerves of Amphioxus can be equivalent to a single nerve in higher vertebrates : (i) Because he finds no connecting branch between two succeeding nerves, and no trace of an anastomosis. (2) Because he finds that each nerve in Amphioxus supplies a complete myotome, and he considers it inadmissible to regard the nerves, which in Amphioxus together supply two myojomes, as equivalent to those which in higher vertebrates supply a single myotome only.
1 From the Jotirnal of Anatomy and Physiology, Vol. X. 1876. - Mem. Acad. Petersbourg, Vol. XIX. 3 Archiv f. mikr. Anatomie, Vol. xn.
Although the agreement as to facts between Langerhans and Stieda is apparently a complete one, yet a critical examination of the statements of these two authors proves that their results, on 'one important point at least, are absolutely contradictory. Stieda, PI. III. fig. 19, represents a longitudinal and horizontal section through the spinal cord which exhibits the nerves arising alternately on the two sides, and represents each myotome supplied by one nerve. In his explanation of the figure he expressly states that the nerves of one plane only (i.e. only those with dorsal or only those with ventral roots) are represented ; so that if all the nerves which issue from the spinal cord had been represented double the number figured must have been present. But since each myotome is supplied by one nerve in the figure, if all the nerves present were represented, each myotome would be supplied by two nerves.
Since Langerhans most emphatically states that only one nerve is present for eacJi myotome, it necessarily follows that he or Stieda has made an important error ; and it is not too much to say that this error is more than sufficient to counterbalance the value of Langerhans' evidence as a confirmation of Stieda's statements.
I commenced my investigations by completely isolating the nervous system of Amphioxus by maceration in nitric acid according to the method recommended by Langerhans 1 . On examining specimens so obtained it appeared that, for the greater length of the cord, the nerves arose alternately on the two sides, as was first stated by Owsjannikow, and subsequently by Stieda and Langerhans ; but to my surprise not a trace could be seen of a difference of level in the origin of the nerves of the same side.
The more carefully the specimens were examined from all points of view, the more certainly was the conclusion forced upon me, that nerves issuing from the ventral corner of the spinal cord, as described by Stieda, had no existence.
Not satisfied by this examination, I also tested the point by means of sections. I carefully made transverse sections of a successfully hardened Amphioxus, through the whole length of the body. There was no difficulty in seeing the dorsal roots in every third section or so, but not a trace of a ventral root was to be seen. There can, I think, be no doubt, that, had ventral roots been present, they must, in some cases at least, have been visible in my sections.
In dealing with questions of this kind it is no doubt difficult to prove a negative; but, since the two methods of investigation employed by me both lead to the same result, I am able to state with considerable confidence that my observations lend no support to the view that the alternate spinal nerves of Amphioxus have their roots attached to the ventral corner of the spinal cord.
How a mistake on this point arose it is not easy to say. All who have worked with Amphioxus must be aware how difficult it is to conserve the animal in a satisfactory state for making sections. The spinal cord, especially, is apt to be distorted in shape, and one of its ventral corners is frequently produced into a horn-like projection terminating in close contact with the sheath. In such cases the connective tissue fibres of the sheath frequently present the appearance of a nerve-like prolongation of the cord ; and for such they might be mistaken if the sections were examined in a superficial manner. It is not, however, easy to believe that, with well conserved specimens, a mistake could be made on this point by so careful and able an investigator as Stieda, especially considering that the histological structure of the spinal nerves is very different from that of the fibrous prolongations of the sheath of the spinal cord.
It only remains for me to suppose that the specimens which Stieda had at his disposal, were so shrunk as to render the origin of the nerves very difficult to determine.
The arrangement of the nerves of Amphioxus, according to my own observations, is as follows.
The anterior end of the central nervous system presents on its left and dorsal side a small pointed projection, into which is prolonged a diverticulum from the dilated anterior ventricle of the brain. This may perhaps be called the olfactory nerve, though clearly of a different character to the other nerves. It was first accurately described by Langerhans 1 .
Vertically below the olfactory nerve there arise two nerves, which issue at the same level from the ventral side of the anterior extremity of the central nervous system. These form the first pair of nerves, and are the only pair which arise from the ventral portion of the cerebro-spinal cord. The two nerves, which form the second pair, arise also opposite each other but from the dorsal side of the cord. The first and second pair of nerves have both been accurately drawn and described by Langerhans : they, together with the olfactory nerve, can easily be seen in nervous systems which have been isolated by maceration.
In the case of the third pair of nerves, the nerve on the right-hand side is situated not quite opposite but slightly behind that on the left. The right nerve of the fourth pair is situated still more behind the left, and, in the case of the fifth pair, the nerve to the right is situated so far behind the left nerve that it occupies a position half-way between the left nerves of the fifth and sixth pairs. In all succeeding nerves the same arrangement holds good, so that they exactly alternate on two sides.
Such is the arrangement carefully determined by me from one specimen. It is possible that it may not be absolutely constant, but the following general statement almost certainly holds good.
All the nerves of Amphioxus, except the first pair, have their roots inserted in the dorsal part of the cord. In the case of the first two pairs the nerves of the two sides arise opposite each other ; in the next few pairs, the nerves on the right-hand side gradually shift backwards : the remaining nerves spring alternately from the two sides of the cord.
For each myotome there is a single nerve, which enters, as in the case of other fishes, the intermuscular septum. This point may easily be determined by means of longitudinal sections, or less easily from an examination of macerated specimens. I agree with Langerhans in denying the existence of ganglia on the roots of the nerves.