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Show 552 MR. J. B. SUTTON ON ATAVISM. [Dec. 21, investigations point to the probability that the finger typically attached in Hipparion to the medial styloid bone was the last to abort. It is certainly noteworthy that in the majority of cases of Polydactyly occurring in Horses it is this digit which reappears most frequently. In the Teratological Gallery in the Museum of tbe College of Surgeons several specimens of Horses' feet are shown with a well-developed second digit. Cheauveau figures1 a specimen preserved in the Veterinary Museum at Lyons ; but the most complete case of its kind is that figured and described in Prof. Marsh's paper2 " On Polydactyle Horses, Recent and Extinct." In this instance the inner digit was present on the four feet. Besides giving an interesting account of extra digits noticed by early writers, this eminent palaeontologist tells us that the more frequent occurrences of extra digits on the manus is precisely what a study of fossil forms of equine mammals would lead us to anticipate. These cases support the view as Gegenbaur points out, that the atavistic parts do not belong to forms palaeontologically remote or systematically far distant. In the Spider-Monkey the thumb is normally absent, or represented merely by a nodule of cartilage or fibrous tissue. Yet on one occasion I dissected an Ateles paniscus which had on each hand a perfectly developed thumb. This condition is not very uncommon. If the parts be dissected in normal specimens, the thumb is represented by a band of cartilage and fibrous tissue attached to the trapezium. The illustrations which have been used were selected merely to give a clear notion of genuine Atavism-the palaeogenetic form. The question of polydactyly is in some instances susceptible of another explanation, which I do not propose to speak of in this paper. Atavism in relation with the Prostate. Leaving the skeleton, attention will now be invited to a remarkable instance of Atavism presented by the prostate. In structure and intrinsic relations this organ presents some exceptional features. It is best regarded as a capsule composed of fibrous and unstriped muscular tissue, mixed with tubular glands, surrounding a recess known as the utriculus masculinus. The utriculus separates the termination of the confluent ducts of the vesiculse seminales and vasa deferentia, whilst it is tunnelled near its anterior aspect by the commencement of the urethra. The glands of the prostate are best considered as tubular alveoli which unite into a smaller number of excretory ducts (about twenty) opening in two depressions of the urethra known as the prostatic sinuses. The tubules are lined by columnar epithelium. The central recess before mentioned has been named the sinus 1 'Anatomie Compare^.' 2 American Journal of Science and Art, vol. xvii., June 1879. |