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The Morphology of the Septum, Hippocampus, and Pallial Commissures in Repliles and Mammals' J

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THE MORPHOLOGY OF THE SEPTUM, HIPPOCAMPUS, AND PALLIAL COMMISSURES IN REPLILES AND MAMMALS' J. B. JOHNSTON Institute of Anatomy, University of Minnesota NINETY-THBEE X'IGURES In the mammalian brain the hippocampus extends from the base of the olfactory peduncle over the corpus callosum and bends down into the temporal lobe. Over the corpus callosug there is a well developed hippocampus in monotremes and mar- supials, whife in higher mammals it is reduced to a sIender ves- tige consisting of the stria longitudinalis and indusium. The telencephdic commissures in monotremes and marsupials form two transverse bundles in the rostra1 wall of the third ventricle. We owe our knowledge of the history of the pallial commissures in mammals chiefly to the work of Elliot Smith. This author states that these commissures are both contained in the lamina terminalis. The upper (dorsal) commissure represents the com- missure hippocampi or psalterium; the lower (ventral) contains the comrnissura anterior and the fibers which serve the functions of the corpus callosum. In mammals as the general pallium grows in extent there is a corresponding increase in the number of corpus callosum fibers. These fibers are transferred from the lower to the upper bundle in the lamina terminalis, in which corpus callosum and psalterium then lie side by side. As the pallium grows the corpus callosum becomes larger, rises up and bends on itself until it finally forms the great arched structure which we know in higher mammals md man. During all this process two changes have taken place in the lamina terminalis. First, it was invaded by cells frDm theneigh- boring medial portion of the olfactory lobe so that the paired 1 Neurological studies, University of Minnesota, no. 18. 37 1 372 J. B. JOHNSTON medial olfactory nuclei came to be connected by a median mass through which the commissures crossed from side to side. The median mass was called by Elliot Smith the commissure bed and the whole body, including the paired gray masses, was called the paraterminal body. Second, the enlargement and arching up of the corpus callosum caused the stretching of the lamina terminalis and the commissure bed. The space within the con- cavity of the callosal arch was filled by a part of the paratermi- nal body. This part of the medial wall is thick in lower forms but the great arching of the corpus callosum in higher mammals has stretched it out into the thin septum pellucidum. This very clear conception of the commissures and their rela- tionships has been generally accepted and has been followed by the writer. Elliot Smith and others have carried this concep- tion into the study of reptiles and have based upon it some homologies in the brains of amphibians and fishes. The writer has approached all questions of brain morphology from a dif- ferent standpoint. He has directed his attention always to the more primitive forms, making the attempt first to see the struc- ture and relationships existing in those brains, with the hope that thereafter it would be possible to see with certainty how the specialized brains of higher forms have been developed from the lower. The method heretofore followed by most workers has been to study man and mammals first, as the animals in which we are directly interested, and then to apply the results to lower vertebrates either as a matter of curiosity or as a means of securing corroborative evidence of their accuracy. The ge- netic method is more tedious but if followed with caution and fidelity to the actual facts it should lead to permanent results. Caution is necessary in the forms chosen for study and the significance accorded to the facts discovered. If we are to use lower forms to throw light upon the human brain, those forms must be chosen for study which are at once the least specialized and most nearly allied to the ancestors of mammals and man. In studying such brains attention must be given to the habits of the animals and the consequent degree of development of each system of nerve centers. SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 373 The work of the writer has shown that the telencephalon of cyclostomes is truly primitive as compared with that of other fkhes. It is believed that the more primitive selachians are the nearest existing allies to the cornmon ancestors of fishes, amphib- ians and reptiles. Among the reptiles those which are regarded by paleontologists as nearest the line of mammalian descent are the chelonia. The writer’s reasoning has been that an understanding of the morphology of the mammalian and human brain must rest on a clear knowledge of the steps in its evolution and of the forces (habits, mechanical factors, etc.) which have been concerned in that evolution. For this purpose we must be able to give a consistent account of the evolution of the brain through the chief groups of vertebrates which stand nearest the line of de- scent of higher mammals : the cyclostomes, selachians, dipnoi, chelonia, monotremes, marsupials, and certain mammals. With this in view the writer, having studied the telencephalon in cyclostomes and selachians, wishes to present here some con- tributions upon reptiles and mammals. Further studies of dip- noi, as affording a bridge between selachians and reptiles, are greatly to be desired. The reason for the above statement of the writer’s method of approach to this problem is that the facts thus far observed in the telencephalon of primitive vertebrates do not agree with the accepted views regarding the mammalian brain. The diffi- culties have to do chiefly with the relations of the paraterminal body, lamina terminalis, commissures and hippocampus. In selachians the pallial commissures are imbedded in a massive roof (called by the writer the primordium hippocampi) and cross through the lamina supraneuroporica, wholly independent of the lamina terminalis and the paraterminal body. The fiber con- nections of the body called primordium hippocampi are so char- acteristic and the line of demarcation between it and the para- terminal body is so clear, that there can be no question of the individuality of the primordiuni hippocampi in selachians and .cyclostomes. The only doubt which has arisen in the use of this term has been whether the body in question may not give 374 J. B. JOHNSTON rise to something more than the hippocampal formation in the mammalian brain (Johnston '10 c, p. 149). With the study of the brains of a number of reptiles and mammals, the conviction has gradually forced itself on the writer that the prevailing ideas regarding the morphology of the medial wall of the telenceph- alon in mammals must be modified in view of the fundamental relations in lower vertebrates. The evidence for this can best be presented by careful comparison of the chief structures in this region in lower and higher vertebrates. For the sake of clearness it must be understood that the term primordium hippocampi is used in the sense in which the writer has employed it in his previous papers. The fitness of this and other terms is discussed in a section at the end of this paper. THE ROOF-PLATE IN THE TELENCEPHALON The anterior end of the brain floor is occupied by the optic chiasma ('09 b). The roof-plate begins at the preoptic recess and extends in a curve around the topographic rostral end of the brain and caudad into the roof of the diencephalon'('10 c). The velum transversum, always present in vertebrates at least in embryos, marks the boundary between diencephalon and telencephalon in the roof ('09 b). The telencephalic roof-plate consists of three portions, the lamina terminalis, lamina supra- neuroporica and tela chorioidea ('11 b, p. 491). The lamina terminalis is formed by the fusion of the lips of the primitive neuropore and is bounded above by the neuroporic recess which marks the point of latest connection of the neural tube with the ectoderm. The lamina supraneuroporica is thickened by com- missural fibers in cyclostomes and by gray matter and fibers in selachians, and even in ganoids and bony fishes where it usually contains no commissures it is thickened and is histologically different from the tela chorioidea ('11 b). From the tela chorioi- dea just rostral to the velum transversum arises the paraphysis. The attachment of the tela chorioidea to the dorsal border of the wall of the telencephalon medium is known as the taenia fornicis ('11 a, p. 6). The taeniae of the two sides converge ros- trally to meet at the point of junction of the tela chorioidea SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 375 with the lamina supraneuroporica in the mid-line. This point has been labelled m in .numerous figures in preceding papers. Since there is an early differentiation between the tela chorioi- dea and the lamina supraneuroporica in the embryo, this point is always easily recognizable in both embryos and adults of all vertebrates. It is one of the fundamental landmarks in the tel- encephalon. Its position has been noted by Elliot Smith ('97 b, p. 52, fig. 23; '97 c, p. 235) and is indicated in figures 4, 5, 6, 7, 9, 35,45, 55, 64 of this paper. This point may be named margo posterior pallii. To complete the identification of lamina terminalis and lamina supraneuroporica in reptiles and mammals it is necessary only to determine the position of the recessus neuroporicus. The neuroporic recess is always situated rostral or dorsal to the anterior commissure. Dorsal to the recess in cyclostomes and selachians is the lamina supraneuroporica with its commis- sures and gray matter as already mentioned. In selachians there is found an external neuroporic recess ('11 a) in the form of a deep pit or a slender canal, through which blood vessels enter the brain near the upper border of the lamina terminalis.
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