CRANIAL NERVES AT EXIT FROM SKULL. 33 



lachrymal groove ; it then passes across the orbit, between 

 the globe of the eye and the inferior and external recti 

 muscles, and is inserted into the external and posterior 

 part of the sclerotic. This is the only muscle of the eye 

 which does not arise from the bottom of the orbit. 



In dissecting the e} r e of a sheep, ox, or calf, the student 

 is often puzzled by an additional muscle, of which he finds 

 no description. It is a suspensor or retractor of the eye- 

 ball, and, with the exception of man and the apes, appears 

 to belong to all the. mammalia ; in most instances it is a 

 quadrifid muscle, but in the ruminantia it coalesces into a 

 single infundibuliform muscle, embracing the optic nerve, 

 and attached anteriorly to the sclerotic, behind the cornea. 



DISSECTION IY. 



CRANIAL NERVES AT THEIR EXIT FROM THE SKULL. 



Paragraphs marked with an asterisk can hardly be verified in an 

 ordinary dissection. 



Soemmering counts the cranial nerves at their points of 

 exit at the base of the skull as twelve pairs, enumerating 

 each nerve separately. Willis makes them but nine, in- 

 cluding in one nerve all the trunks contained in the same 

 aperture of the skull. The latter division is the one 

 adopted. 



The cranial nerves, as they enter their foramina, are in- 

 vested by a process of the dura mater, which constitutes a 

 sheath for them ; the pia mater also is prolonged upon 

 them, but both membranes are soon lost in the surrounding 

 tissues. The arachnoid is reflected backward upon the 

 internal surface of the cranial cavity. 



The FIRST, or OLFACTORY NERVE, is soft and pulpy, and 

 is often lost in the manipulation of removing the brain ; it 

 lies upon the cribriform plate of the ethmoid bone, and 

 toward its extremit}' assumes a bulbous shape ; it sends a 

 large number of fine filaments, through the foramina of the 

 bone beneath, to supply the mucous membrane of the nasal 

 fossa?. 



The SECOND, or OPTIC NERVE, diverging from its com- 

 missure, as the conjunction with its fellow is called, enters 



