CRANIOLOGICAL DIFFERENCES BETWEEN THE ASS AND THE HORSE. 407 
loop is often greater than the forward one. The external channel is much less 
deep in the ass than in the horse. 
To discriminate among whole skulls, Lesbre finds by comparison, empirically, 
the method of drawing a connecting line between the tuber molaris and the fossa 
articularis ossis zygomaticr; if the prolongation of this line strikes the crista occipi- 
talis, or anterior to this cuts the facial plane, the animal is an ass; if this line passes 
posterior to the crista occipitalis it is a horse. 
Tscherski* places much dependence on the construction of the nasal bones 
and on the form of the suture between the lacrimal and the nasal bone. ‘This, 
he says, is in the ass always curved outward in bow shape, but is straight in the 
horse. I find that this relation is very dependent on the breadth of the forehead. 
The foramina infra-orbitalia stand nearer (to the naso-maxillar suture) in the ass 
than in the horse. He says, further, that in the horse the processus zygomatici 
ossis frontalis is three-cornered, but in the ass oval and compressed. 
Salenskif asserts that Tscherski’s criteria are valuable and to the point, 
especially the form of the suture between the lacrimal and the nasal bone, but that 
a difference in the cross-section of the processus zygomatici ossis frontalis can not 
be verified in the ass and horse. On the other hand, Salenski proposes another 
method, which he considers extremely constant: the lower edge of the lower 
jaw is in the ass curved and furnished with protuberances, while in the horse 
it is smooth and straight. Unfortunately I can not agree with Salenski, as I 
consider this criterion of the lower jaw very variable, eminently dependent on the 
age of the individual; one has only to compare the lower jaw-bone of the horse 
of Auvernier with the one of exactly similar shape in the ass from Abadieh (plate 
87, figs. 1 and 2). 
So far the authors. It is now time to investigate the conditioning factors 
that influence the shaping of the skull of the ass and of the horse. Starting with 
the law, which I have proved elsewhere,{ that the skull is a flexible product 
of the influence of skin and muscle, the longer ear and its generally different 
position seems to be decisive for the head of the ass. The greater burden imposed 
by the external ear must have had as a consequence a change in the musculature 
of the ear, which in turn, as is shown by Anthony$§ and Toldt| exerts its influence 
on the bones. Indeed the whole ear-musculature is different in the ass, as I was 
able to observe in agreement with Kirsten,{[ on the only fresh head that I could 
obtain. 


* J. D. Tscherski, Wissensch. Resultate d. Neusiberischen Expedition. Mém. Acad. Imp. St. 
Pétersbourg, t. XL, 1893. : 
+W. Salenski, Equus przewalski Polyakof. Wissensch. Resultate d. von Przewalski nach Central 
Asien unternommenen Reisen. Zoolog. Teil, Bd. 1, Abt. 2, Lief 1, pp. 27-47, St. Petersburg, 1902. 
tExperimentelle Studien ueber die Morphogenie des Schaedels der Cavicornia, Vierteljahrschr. 
Naturf. Gesellsch. Zitirich, 1903, pp. 360-374. 
§R. Anthony, Etudes experimentales sur la morphogénie des os. Modification craniennes consecu- 
tives a l’oblation d’un crotaphyte chez le chien. Journal de Physiologie et de Pathologie Gén., No. 2, 
Mars, 1903. 4 ed ol he 
|| R. Toldt, Asymetrische Ausbildung der Schlaefenmuskeln bei einem Fuchs infolge einseitiger Kau- 
tatigkeit. Zoolog. Anzeiger, 1905, Bd. xxix, No. 6. : ; 
{F. Kirsten, Untersuchung ueber die Ohrmuskulatur verschiedener Saugetiere. Inaug. Diss., Bonn, 
1902. 
