DISSECTION OF THE DOG 155 



Dissection. — Reflect the longissimus capitis et atlantis muscles. Remove 

 the caudal oblique muscle of the head and the intertransverse muscles in 

 order to show the vertebral vessels and the second cervical nerve. 



A. vertebralis. — The vertebral artery is a branch of the subclavian. 

 After leaving the chest, the artery enters the interval between the longus colli 

 and scalenus muscles and gains the transverse foramen of the sixth cervical 

 vertebra. After traversing the foramina in the transverse processes of the 

 cervical vertebrae from the sixth to the first, a union is effected under the wing 

 of the atlas with a branch of the occipital artery. 



The dissection of the face should be commenced by an examination of the 

 external parts of the eye. 



PAXPEBRiE. — The eyelids, using the term in the more usual sense, are two 

 in number : upper (palpebra superior) and lower (palpebra inferior) ; but to 

 these may be added a much less obvious third eyelid (palpebra tertia). The 

 upper and lower eyelids may be described as membranous curtains arranged 

 for the protection of the eye. The upper lid is the larger and the more movable, 

 and is provided with a special muscle, the m. levator palpebrce superioris, by 

 which it is raised. Each lid has an outer surface (facies anterior palpebrae) 

 covered with hair-bearing skin, and an inner surface (facies posterior palpebrse) 

 applied to the eyeball and rendered smooth by the conjunctiva, a membrane 

 continued from the lid over the front part of the eyeball. 



The free borders of the lids (limbi palpebrales) bound the palpebral fissure 

 (rima palpebrarum), which is in the form of a narrow slit when the lids are 

 closed, but assumes an elliptical outline when the eye is open. The free border 

 of the upper eyelid carries a row of long and strong hairs. The two lids are 

 joined at the lateral and medial commissures (commissurae palpebrarum 

 lateralis et medialis) . Both are acute, and in the neighbourhood of the medial 

 commissure the eyelids circumscribe a space (lacus lacrimalis) in which is a 

 rounded body, the lachrymal caruncle (caruncula lacrimalis). On the margins 

 of the lids near the medial commissure are the small puncta lacrimalia, upper 

 and lower, leading into the lachrymal ducts (ductus lacrimales), by which the 

 tears are drained from the eye. 



Each eyelid contains a framework of fibrous tissue, the tarsus, much better 

 developed in the upper than in the lower lid. A sphincter muscle (m. orbicu- 

 laris oculi) is associated with the eyelids, and can be exposed by the removal 

 of the skin over them. In addition, the upper lid and the region of the eyebrow 

 contain muscular fibres which corrugate the eyebrow (m. corrugator supercilii) . 



The third eyelid consists of a pigmented semilunar fold of conjunctiva, the 

 membrana nictitans, easily demonstrated near the medial commissure, and a 



