MUSCLES OF THE ORBIT. 179 



neck. It is the only representative of a subcutaneous group of muscles largely 

 developed in most mammals, by which very varied movements of the skin and some 

 superficial parts may be given, as, for example, when the horse communicates a rapid 

 motion to the skin to free itself from insects, or the dog shakes off the water after 

 swimming, or the hedgehog elevates its spines ; and which is known collectively as 

 the panniculus carnosus. 



MUSCLES OF THE ORBIT. 



In this group will be described seven muscles, namely, the elevator of 

 the upper eyelid already alluded to, and six muscles of the eyeball, viz, 

 the four straight and the two oblique muscles. Of these muscles, the inferior 

 oblique alone is confined to the fore part of the orbit ; all the rest take their 

 origin at its back part, and pass longitudinally forwards to their insertions 

 in front. 



The levator palpebrce superioris is a slender muscle, which arises, pointed 

 and tendinous, above and in front of the margin of the optic foramen, and, 

 passing forwards over the eyeball, ends in a fibrous expansion inserted into 

 the anterior surface of the tarsal cartilage of the upper eyelid. 



Between this muscle and the roof of the orbit are situated the fourth and frontal 

 nerves, and beneath it are the superior rectus muscle and the globe of the eye. In 

 the lid, it is placed behind the palpebral ligament, close to the membrana 

 conjunctiva. 



The four straight muscles of the eye surround at their origin the optic 

 nerve, and, passing forwards from that point, are inserted into the front of 

 the globe of the eye at four opposite equidistant parts, by delicate expanded 

 tendons which become blended into one at their termination. 



The superior rectus arises close in front of the foramen opticum, and 

 beneath the levator palpebrse ; the inferior rectus, internal rectus, and 

 external rectus are united in a common tendinous attachment around the 

 circumference of the optic foramen, except above. But the external rectus 

 differs from the others in having two heads of origin : the upper head unites 

 with that of the superior rectus, the second head arises from a bony point 

 on the lower margin of the sphenoidal fissure close to its wider inner end ; 

 and other fibres are implanted into a fibrous band between the heads of 

 origin. The four recti thus attached posteriorly, pass forwards, one above, 

 one below, and one on each side of the eyeball, becoming flattened as 

 they lie in contact with it, and are inserted by short membranous tendons 

 into the fore part of the sclerotic coat, at an average distance of four lines 

 from the margin of the cornea. 



In length and breadth there are some differences among these muscles. The 

 external rectus exceeds the internal one in length. On the other hand, the internal 

 rectus has some advantage in width, being broader than any ; and the superior 

 appears rather the narrowest of all. Between the heads of the external rectus is a 

 narrow interval, which gives transmission to the third and sixth nerves and the nasal 

 branch of the fifth nerve, with the ophthalmic vein. 



The superior oblique or trochlearis is a narrow elongated muscle, placed 

 at the upper and inner part of the orbit, internally to the levator palpebrae. 

 It arises about a line in front of the inner part of the optic foramen ; 

 thence it proceeds towards the internal angle of the orbit, and terminates in 

 a round tendon which passes through a fibre-cartilaginous ring or pulley 



N 2 



