HEAD AND NECK OF THE HORSE 33 



consists of a well-marked band of fibres running obliquely, immedi- 

 ately underneath the skin, to the angle of the mouth, where they blend 

 with those of the buccinator and orbicular muscles. 



M. ORBICULARIS OCULI. — The orbicular muscle of the eyelids 

 consists of fibres circularly disposed so as to act as a sphincter of 

 the palpebral fissure. The peripheral limit of the muscle is not 

 well defined. A few fibres are attached to the oral lachrymal 

 tubercle of the lachrymal bone, but otherwise there is no direct 

 attachment to bone. 



M. CORRUGATOR SUPERCiLll.i — A feebly developed and thin 

 muscle springs from the surface of the frontal bone and merges 

 into the orbicular muscles in the upper eyelid. 



At this stage the dissector sliould make a preliminary examination 

 of the lips and nostrils before proceeding to the removal of the skin 

 from these structures. 



The two lips (labia oris) are not alike. The upper (labium superius), 

 attached to the incisive bones, is the better developed and the more 

 moveable. It is, moreover, marked in the middle line by a shallow and 

 ill-defined groove, the philtrum. The lower lip (labium inferius) is 

 connected with that part of the mandible that carries the incisor and 

 canine teeth. Inferiorly it merges into the chin (mentum), a rounded 

 prominence composed of muscle, fat and fibrous tissue. The opening 

 between the lips (rima oris) terminates on each side at the angle of the 

 mouth (angulus oris) where the upper and lower lips are connected by 

 the commlssiires (commissurse labiarum). 



The nostril (apertura nasi externa) is a crescentic opening with 

 medial and laterial ivings (alse nasi). The medial wing projects and 

 has a basis formed by the lamina of the alar cartilage — a structure that 

 will be exposed at a later stage of the dissection. The thinner, concave 

 lateral wing is more flexible and consists of skin enclosing muscle and 

 fibrous tissue. The two wings meet at a sharp upper commissure. The 

 lower commissure, on the contrary, is rounded, its form being governed 

 by the curved cornu of the alar cartilage. If the finger or an instru- 

 ment be thrust into the upper angle of the nostril, it does not enter the 

 nasal cavity, but finds its way into a blind diverticuluni of the nostril 

 (diverticulum nasi). The diverticulum, some five or six centimetres in 

 length, extends backwards to the angle of union of the free border of 

 the nasal bone and the nasal process of the incisive bone. Its interior 

 is lined by a fine, pigmented skin, almost entirely devoid of hairs. 



The wings of the nostril should be separated as widely as possible, 



^ Superciiiuvi { = super, a,hove + cilium, the eyelid) [L.], the eyebrow. 

 3 



