The brothers Wenzel figure in their excellent plates the various conditions of the posterior cornu and hippocampus minor to which they refer; and it is remarkable that the brain which they have selected as exemplifying the absence of the hippocampus minor on both sides, Tab. v., Fig. 1, is said to be "ex triginta annorum æthiope," while the most remarkably developed hippocampus, Tab. vii., Pig. 3, is "ex septem annorum puero."
The work whence these extracts are taken is contained in the libraries both of the College of Surgeons and of the Royal Society; but, even if it were inaccessible, a well-known and more modern writer fully bears out the doctrine it contains. I refer to Longet,[1] who states that, in the human brain, "the posterior cornu is found of very different lengths and breadths. I have found brains in which it extended up to within a few millimetres of the surface of the posterior lobe, and others in which it ended at more than three centimetres therefrom."
The same excellent authority, in describing the posterior cornu of the lateral ventricle, says:—
"Its inner and lower wall is raised by a convolution which forms a more or less distinct, and at times, double projection into the cavity itself. This projection (Hippocampus minor, eminentia unciformis, calliculus, unguis, calcar avis) was well described by Morand, and after him was called the 'Spur of Morand'—'Ergot de Morand.'
"The Hippocampus minor exhibits differences in its form and circumference, as Greding has stated; usually it is bent on itself, arched forwards and outwards, sometimes narrow and long, sometimes broader. Very frequently it is smooth, at other times it exhibits many fissures and small enlargements, especially posteriorly; or it may be divided by a longitudinal cleft into two halves, the upper of which is almost always larger than the lower. Its dimensions are by no means directly proportional to the development of the posterior lobe. In the same subject it may be very distinct upon the one side, and yet be hardly perceptible upon the other. For the rest I can certify that, in spite of Meckel's[2] assertion to the contrary, it is not always present. My own observations agree with those of Wenzel, who, among fifty-one subjects that he examined with express reference to this point, found three in which the hippocampus was absent upon both sides, and two in which every trace of it was absent upon one side only."
To allow a structural character totally absent in six per cent, of the members of any group to stand as part of the definition of that group, considered as a sub-class, would be a very hazardous proceeding. But, is it true that the hippocampus minor is altogether absent in the highest apes? I suspect that Tiedemann is responsible for the not unfrequently admitted doctrine that it is; for, in the "Icones" he writes:—
"Pedes hippocampi minores vel ungues, vel calcaria avis, quæ a posteriore corporis callosi margine tanquam processus duo medullares proficiscuntur, inque fundo cornu posterioris plicas graciles et retroflexas formant, in cerebro simiarum desunt; nec in cerebro aliorum a me examinatorum mammalium occurrunt. Homini ergo proprii sunt."
- ↑ German edition, by Hein, under the title, Anatomie und Physiologie des Nervensystems des Menschen und der Wirbelthiere, 1847, Bd. i., p. 463.
- ↑ Dr. Hein here adds: "What Meckel says is that he himself never failed to find the hippocampus minor, but that he by no means wishes to throw doubts on Wenzel's statements;" and on reference to Meckel's work, I find this to be quite correct.
