DISSECTION OF THE DOG 159 



superior labial artery (a. labialis superior), which ramify in the parts indicated 

 by their names. 



A. infraorbitalis. — The infraorbital artery leaves the homonymous 

 foramen along with the large nerves of the same name, and terminates in 

 dorsal nasal (a. nasi dorsalis) and lateral nasal (a. nasi lateralis) branches. 



A. transversa faciei. — The transverse facial artery, a branch of the 

 superficial temporal, is generally a very small vessel which crosses the surface 

 of the masseter muscle. 



V. facialis et v. maxillaeis externa. — The facial vein is formed by 

 the union of the angular vein of the eye and the dorsal nasal vein. Running 

 down the face, it receives the superior labial, reflex, buccinator, and inferior 

 labial veins, and becomes the external maxillary vein on a level with the 

 inferior border of the mandible. Instead of joining the facial vein directly, 

 the buccinator may unite with the inferior labial. 



The external maxillary vein will be found in the groove between the mas- 

 seter and digastric muscles. It runs backwards ventral to the submaxillary 

 gland, and ends by uniting with the internal maxillary vein to form the external 

 jugular. The main collateral tributaries of the external maxillary vein are 

 the deep facial and the lingual. The former appears at the oral border of 

 the masseter muscle. The lingual vein joins the external maxillary just before 

 this flows into the jugular. A transverse vessel, on a level with the body of the 

 hyoid bone, forms a link between the lingual veins of the two sides of the head. 



Dissection. — The parotid gland must now be removed with great care. When 

 this has been done, the origin of those branches of the facial nerve which 

 have recently been examined in connection with the face will be exposed. 



M. masseter. — The powerful masseter muscle completely fills the con- 

 siderable depression ventral to the zygomatic arch. Very tendinous in structure, 

 its origin is from the whole of the ventral border and the immediately adjacent 

 medial surface of the zygomatic processes of the zygomatic and temporal bones. 

 On account of a difference in the direction of its fibres at different depths, the 

 muscle may be divided into three imperfectly separated strata. The most 

 superficial fibres are caudo- ventral in direction ; the middle stratum has 

 practically vertical fibres ; and in the deepest stratum their direction is ventral 

 and oral. 



The masseter muscle is inserted into the whole of the fossa on the lateral 

 surface of the mandible, the ridges bounding the fossa, the angular process, 

 and a ridge continued in an oral direction from this process. 



In the dissection of the masseter muscle care must be exercised to preserve 

 its nerve of supply — the masseteric nerve (n. massetericus) — a branch of the 

 mandibular nerve, which reaches the muscle by way of the mandibular notch, 

 and is most easily found between the middle and deep strata of the muscle. 



Dissection. — The temporal muscle is covered by a strong temporal fascia 



