Lectures on the Elements of Comparative Anatomy

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100 ON CLASSIFICATION. In fact, even if the posterior cor~u had not been demon-t d ( 't J1stra e as 1 as now been) in the .b rain of numerous genera of A pes, tl u·s one example would sufficiently. demonstrate the asser-tion, that thA posterior cornu is "peeuha.r to the genus Homo," to be simply untrue. . . . . Lastly, as regards the hippocampus mi~1or-whiCh IS al~o said t be "peculiar to man" -that structure I , as you perceive, as d~ tinct in the Oran<Y's as in the man's brain, o that the third IS o l " ' term of the definition of the ".Archencepha a 1s as contradic-tory to plain fact as the other two (Fig. 40) .. Even were the posterior lobe, the posten or cornu, and the hippocampus minor peculiar to n1 a,~ ' ~s suppos~d b~ tho definer of the sub-class ".Archencephala, 1nstead of bmng, as they really are, structures far better developed in son1e of the lower apes than in him, their classificatory value would ~e extremely doubtful, seeing that they are among the 1nost vanable of struc­tures in the human brain. The casts upon the table of a Tartar's and of an Australian brah1-ca e will demonstrate to you how in­significant may be the projection of the po .. tori or lobe i~ one man and how great it may be in another. vVhile the practical anato­mists and demonstrators whom I address will be familiar with the singular variability of the posterior cornu an:l the hippocampus minor-structures which, without any assignable cause, or noticeable modification of the structure, or of the functions, of the brain, may present every degree of development, from absence to great size. So little, indeed, is any zoological value to be attached to such a character as the degree of projection of the posterior lobe, that closely allied apes present us with most singular .dif­ferences in this respect. Thus the group of South American monkeys ·which comprises the Squirrel monkey (Ohrysothrix), the posterior lobes of whose brain project beyond the cerebellum far more than they do in man, contains al o the liowling mon~cey (Mycetes), in which the posterior lobes cannot be said t.o proJect at all. And within the last two days, 1\Ir. Flower has discovered (and the cast upon the table enables me to demonstrate t~e fa.c~ to you) that in, at any rate, one species of Gibbon, the S1amang (Hylobates Syndactylus) the cerebellum projects behind the pos- THE HUMAN PLACENTA. 101 terior lobes, while, in the three other genera of anthropoid apes, the posterior lobes of the cerebrum project behind the cere­bellum.* The latest form of the "cerebral" classification of the Mam­malia having thus been shown to be devoid of any sound foundation, I proceed to inquire whether the c: placental" classification does, or does not, stand upon a more secure basis, if we take, not merely, with Milne-Edwards, the form of the placenta, but with Von Baer and Esehricht, its structure, into account. It is a well-established fact that two very distinct types of placenta are to be met with in the Monodelphia, and that, at the present moment, we have no knowledge of any transitional forms between these two types. The first of these types is that exhibited by the human placenta, the second by that of the pig or horse. From the commencement of gestation, the superficial sub­stance of the mucous membrane of the human uterus undergoes a rapid growth and textural modification, becoming converted into the so-called "decidua." While the ovum is yet small, this dec,£dua is separable into three portions,-the decidua vera, which lines the general cavity of the uterus; the decidua rejlema, which immediately invests the ovum; and the decidua serotina, a layer of especial thickness, developed in contiguity with those chorionic villi which persist and become converted into the fretal placenta. The decidua reflexa may be regarded as an outgrowth of the decidua vera; the decidua serotina as a special development of a part of the decidua vera. At :first, the villi of the chorion are loosely implanted into correspond­ing depressions of the decidua ; but, eventually, the chorionic part of the placenta becomes closely united with, and bound to, the uterine decidua, so that the fretal and maternal structures form one inseparable mass. In the meanwhile, the deeper substance of the uterine * See Mr. Flower's paper "On the Brain of the Siamang," Natural History R eview, April, 1863. "This peculiarity of the Siamaug's brain is due to two causes-firstly, the large development of the cerebellum; secondly, and I sllall afterwarus show, mainly, to tho actual shortness of the posterior or occipitai lobe of the cerebrum."-L. c., p. 282.