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 the College of 
Surgeons several specimens of Horses’ feet are shown with a well- 
developed second digit. Cheauveau figures * 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 paper* “On 
Polydactyle Horses, Recent and Extinct.” In this instance the 
inner digit was present on the four feet. Besides giving an in- 
teresting account of extra digits noticed by early writers, this eminent 
paleontologist 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 palzeontologically remote or 
systematically far distant. 
In the Spider-Monkey the thumb is normally absent, or repre- 
sented merely by a nodule of cartilage or fibrous tissue. Yet on 
one occasion J 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 repre- 
sented 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 palogenetic 
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 wiil now be invited to a re- 
markable instance of Atavism presented by the prostate. In struc- 
ture 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 vesiculze 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 
} «Anatomie Comparée.’ 
2 American Journal of Science and Art, yol. xvii., June 1879. 
7]? 
