On the commissures of the cerebral hemispheres of the Marsupialia and Monotremata as compared with those of the placental mammals / by William Henry Flower.

  • Flower, William Henry, 1831-1899.
Date:
[1865]
Catalogue details

Licence: Public Domain Mark

Credit: On the commissures of the cerebral hemispheres of the Marsupialia and Monotremata as compared with those of the placental mammals / by William Henry Flower. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    Now the corpus callosum breaks through just at the limit between these two divi- sions, and by its further growth backwards, the upper division comes to lie on its outer surface and is converted into the stria alba Lancisi and stria obtecta of the corpus cal- losum, and into the fascia dentata of the hippocampus major; whilst the inferior or inner arch, with its longitudinal fibres, forms the fornix and septum (scheulewcmd). The fornix is thus, as was known to Arnold and Retzius, a transformation of the upper margin of the transverse fissure. The lower margin of the fissure is formed into the taenia semicircularis or stria cornea, which, as is well known, is connected at each end with the extremities of the fornix. It will be seen from the preceding observations that the anterior perpendicular part of the fornix is originally united with the corresponding part of the other side, and the body of the fornix developes itself out of the uppermost part of this spot, adjoining the primitive corpus callosum. Lower down the parts sepa- rate and then resolve themselves into the columnee fornicis, or anterior crura, and the two halves of the septum lucidum, the ventricle of which is thus no primitive formation. In this part also originates, not by growing together from opposite sides, but by histological differentiation, the anterior commissure (F), which is evident a short time before the corpus callosum. The septum lucidum and body of the fornix, in the beginning very small, gradually increase in extent with the development of the corpus callosum. According to Schmidt, the opinion formerly entertained that the genu of the corpus callosum was the part first formed, and that the hinder part developed afterwards, is not correct. The rudimentary corpus callosum on its first appearance already contains the elements of all its subsequent parts, as from the very first, fibres radiate from it into the hinder and middle, as well as the anterior lobes, and the intimate connexion of the former with the posterior crura of the fornix can already be recognized. It increases, with the rest of the hemisphere, chiefly in longitudinal extent, spreading both backwards and forwards from the point of its first appearance, but principally in the former direc- tion. The curved part in front, called the genu, is not formed until the end of the fifth month, and about a month later, the thickening and extension of the hinder end over the corpora quadrigemina gives the permanent form to this part of the brain. I will next proceed to trace the modifications of the parts of the brain above indicated, in certain of the placental mammalia. The preparations from which the figures are taken were all made in the same manner as that adopted in the case of the human brain, viz., (I.) a vertical longitudinal section in the middle line, exhibiting the inner surface of a single (the right) hemisphere, the thalamus opticus and crus having been removed so as to show clearly the whole surface with the parts forming the upper boundary of the ventricular aperture; (II.) a vertical transverse section through the middle of the anterior commissure. The Sheep.—In the longitudinal section of the sheep’s brain (Plate XXXVII. fig. 1), the elongated narrow corpus callosum (B) is seen lying in a line nearly horizontal, or corresponding with the long axis of the hemisphere; slightly concave in the middle