376 SENSITIVE SYSTEM. 



mechanical or chemical irritation ; and in the human subject, 

 weeping is induced by excessive pain, and by certain emotions 

 of the mind, particularly grief. Augmented secretion of tears 

 may be the effect of inffamniatory action in the gland itself; but 

 it is more frequently, perhaps, the result of sympathy. 



The reiyiaining muscle of the eyelids — the levator palpebrse su- 

 perioris internus — and also the 



Muscles of the Eyeball, 

 Having been already described (at page 87), we pass on to the 

 eye itself; finishing the description of the Appendages with a few 



Concluding Observations. 



Contents of the Orbit. — In addition to the contents of the orbit 

 already spoken of, viz. the lachrymal gland, the levator palpe- 

 brse superioris, and the seven muscles belonging to the eyeball, 

 there are (besides the globe of the eye itself) some vessels and 

 several important nerves met with therein. There is the optic 

 nerve, sheathed in dura mater, in the middle, entering at the 

 foramen opticum, and penetrating the back of the globe. The 

 nerve of the third pair, supplying three of the recti, the inferior 

 oblique, and the levator palpebrae, besides contributing to the 

 formation of the lenticular ganglion. The nerve of the fourth 

 pair, running to the trochlearis. The nerve of the sixth pair, to 

 the rectus exterims. And the ophthalmic artery, whose ramifi- 

 cations are distributed to the globe and the various parts con- 

 nected with it ; and also the ophthalmic vein, which returns the 

 blood into the cavernous sinus. 



These several parts are cushioned and packed in an abun- 

 dance of soft adeps, which serves to connect them together, and 

 at the same time accurately fills up the interspaces. Being in a 

 state of semi-fluidity during life, this substance readily accom- 

 modates itself to the various movements of the globe, which or- 

 dinarily it bolsters forward in front of the orbit; although it is 

 every now and then forced to one side, and then drives before it 

 the membrana nictitans. That this is one of its principal pur- 

 poses is shewn by there being comparatively much less of it in 

 man. In cases of extreme emaciation, part of this fat becomes 

 absorbed, producing that appearance called sunk-eyed. 



Ocular Case. —The orbit is lined with a dense, firm, fibrous 

 ocular sheath or case, having the appearance of dura mater, of 

 which it is said to be a continuation. It possesses fibrous at- 

 tachments to the bony inequalities and foramina at the bottom 

 of the orbit, and by several threads is fastened to the back or 



