THE HEAD AND NECK 293 



platysma, the pars zygomatica platysmatis, has acquired an insertion on the 

 zygomatic bone and is more or less separated from the main muscle. A deeper 

 transverse layer, the sphincter profundus, is also distinguishable. 



In removing the skin of the upper and lower eyelids, the dis- 

 sector may observe the orbicularis oculi. This is a thin, somewhat 

 ring-like band of muscle lying directly on the inner surface of the 

 skin and forming a sphincter round both eyelids, which are closed 

 by its contraction. The muscle fibres are concentrated at the 

 anterior and, more particularly, at the posterior angles. The 

 antagonistic action of raising the upper eyelid is accomplished by 

 a muscle (levator palpebrae superioris) which arises from the orbital 

 wall and, at this stage of dissection, is concealed by the projecting 

 supraorbital process, while the depressor palpebrae inferioris 

 consists of an extremely delicate group of muscle fibres described 

 as originating on the zygomatic process of the maxilla and inserted 

 in the anterior part of the lower eyelid. 



Round the mouth, a very thin band of fibres on the skin constitutes the 

 orbicularis oris, which in the rabbit is inconspicuous and forms a ring inter- 

 rupted dorsally on account of the cleft upper lip. It is a special portion of the 

 sphincter profundus mentioned above. 



Small lymph glands of irregular occurrence are usually found ventral to 

 the mandible. These receive the flow from subcutaneous lymphatic vessels and 

 are drained by other vessels into the superficial lymph glands of the neck, of 

 which usually two or three are associated with the external jugular veins and 

 give rise to the jugular trunks. These last empty into the veins near the junction 

 of the jugulars with the subclavians. 



1. On the lateral surface of the head, the following structures may 

 be made out after removal of the platysma without further dis- 

 section beyond the clearing of a little fascia. 



{a) The parotid gland (gl. parotis), a diffuse, white or brownish 

 gland lying immediately behind the angle of the mandible. 

 It expands dorsally to cover the lateral aspect of the 

 base of the external ear and ventrally beneath the 

 mandible (Fig. 46). Its duct (d. parotideus) passes forward 

 across the lateral surface of^the masseter muscle (c) in close 

 association with the branches of the facial nerve (Fig. 107) 

 and, perforating the mucous membrane of the cheek, opens 

 into the oral cavity opposite the last upper molar tooth. 



