THE HEAD. 83 



bone ; above and in front, by the frontal bone and its process ; externally, by 

 the malar bone. If its two largest diameters are measured, it is remarked that 

 this opening is scarcely ever regularly circular, its width varying from above to 

 below or from without to within. With nine Horses' heads of various ages and 

 breeds, equal diameters were found in only one, the other eight being unequal ; 

 of these, the vertical diameter of the orbit predominated in five, and was least in 

 three. 



In the Ass, as a general rule the vertical diameter is smallest, the relation 

 between the two diameters varying from ro9 to 1"15. Otherwise, the entrance 

 to the orbit is irregularly square, and the orbital process which covers it is much 

 wider and more salient than in the Horse — as Lecoq asserted, and as the 

 observations of Goubaux and Sanson have confirmed. We have also noticed 

 these differences, and they are not the only ones which permit the skeleton of 

 the Ass to be recognized ; for there have been already cited those of the spine 

 and bones of the head — such as the articular depression surrounding the basilar 

 process, the vascular furrow on the styloid process, the position of the lachrymal 

 tubercle, and the conical eminence surmounting the premaxillary symphysis, 

 above the incisive foramen ; others will be noted hereafter (Figs. 51, 52). 



With regard to the cavity of the orbit, it is separated from the maxillary 

 hiatus and the temporal fossa by two linear imprints, diverging forwards, to 

 which the ocular sheath is attached. It presents, on its floor, the upper orifice 

 of the lachrymal canal, the lachrymal fossa, where the small oblique muscle of the 

 eye has its fixed insertion ; and within this, but higher, the little depression for 

 the bend of the great oblique muscle of the eye. 



3. Temporal region. — This region is more extensive than the preceding, and 

 is composed of three principal parts — the temporal fossa, zygomatic arch, and 

 petrous portion of the temporal bone. 



The temporal fossa surmoimts the orbit, from which it is incompletely 

 separated in Solipeds and Ruminants by the orbital arch ; in the other domestic 

 animals, this arch is incomplete in such a way, that in the skull the temporal 

 fossa is confounded for the greater part with the orbit. Situated obliquely down- 

 wards and outwards on the sides of the cranium, the temporal fossa is oval in 

 shape, and bounded inwardly by the occipital or temporal crest, outwardly by 

 the anterior border and longitudinal root of the zygomatic process. It lodges 

 the temporalis muscle ; consequently, its width in our animals is proportionate to 

 the power of that muscle. It is studded with muscular imprints, and has several 

 vascular foramina which enter the parieto-temporal canal. 



The zygomcitic arch is formed as if by a loop thrown from the cranium on to 

 the face, outside the temporal fossa and the orbit. It is constituted by the 

 zygomatic process of the temporal bone and the malar bone, which latter 

 prolongs it to the maxillary region. 



The tuberosity of the temporal bone shows, outwardly, the external auditory 

 canal, usually larger in the Ass than the Horse. Between this orifice and the 

 supra-condyloid eminence is the opening of the parieto-temporal canal ; and 

 behind the latter are several irregular ridges, one of them being the hyoid 

 process. The external face of the tympanic case is also studded with some 

 styloid prolotiyations, one of which, more developed than the others, serves for 

 the insertion of the peristaphyline muscles. Above the hyoid process is the 

 mastoid process, and between these two parts is the external orifice of the aqueduct 

 of Fallopius. From the mastoid process extends the mastoid crest, the summit of 



