ANATOMY AND PHYSIOLOGY OF PATELLA VULGATA. 627 



The cerebral ganglia are large irregular nervous masses, situated deep down, 

 just at the base of the tentacles ; the visceral and pedal ganglia lie close 

 together just at the anterior edge of the floor of the visceral cavity. The 

 cerebral ganglia are found to give off (a) nerves and (b) commissures. The 

 nerves are four in number, on either side. (The nerves of the left cerebral 

 ganglia only are here described, those of the right being precisely similar.) 

 First, the tentacular nerve, which arises from the anterior left corner, and 

 passes directly into the nuchal tentacle. This nerve very soon divides, giving 

 off numerous branches to the muscles and skin of the tentacle. Just at its base 

 anteriorly arises a small nerve, the cutaneous, supplying the skin of the neck 

 and snout. Immediately behind the origin of the tentacular nerve is the optic 

 nerve, which is not a branch (as might be supposed) of the tentacular, but is an 

 entirely distinct nerve, going directly to the eye. The nerve divides repeatedly 

 ere reaching the eye, and loses itself in a nervous plexus, immediately behind 

 the retina. All these nerves spring from the outer aspect of the cerebral 

 ganglion. On its inner aspect, immediately opposite to the origin of the optic 

 nerve, is a small nerve supplying the muscles of the pharynx, and which may 

 be termed therefore the posterior lateral pharyngeal nerve. In addition to 

 these nerves there are a number of commissures uniting the cerebral ganglia to 

 the rest of the system. The anterior end of either ganglion gives off two com- 

 missures, one passing in front of the buccal mass, and easy to find, the other 

 passing vertically up the side of the same, and more or less involved in muscle 

 and connective tissue. The former is a large thick white commissure which 

 runs in front of the oesophagus, far forward. It unites the two cerebral 

 ganglia. After leaving either ganglion the commissure is slightly swollen, 

 and at that point it gives off a nerve, which, since it supplies the anterior 

 muscle of the pharynx, may be termed the anterior lateral pharyngeal nerve. 

 In front the commissure gives off many small nerves which supply the lips. 

 At the point where the anterior pharyngeal is given off a nerve loop encloses 

 the commissure, coming from below upwards and passing backwards. It is 

 not connected in any way with the commissure. Following now the second 

 commissure, which springs from the cerebral ganglion, we find it mounts 

 the side of buccal mass, and becomes united to a small ganglion lying 

 at the side of the pharynx and on the top of the muscles of the infraradular 

 sheet. This ganglion is one of four which lie at the angles of a square formed 

 by the commissures which unite them, i.e., along either side of the pharynx. 

 On the top of the muscles of the infraradular sheet there are two ganglia 

 united to each other and to their fellows on the opposite side. These ganglia 

 are the superior, anterior, and superior posterior buccal ganglia respectively. 

 To the anterior ganglion on either side is united the end of the loop spoken 

 of above as enclosing the cerebral commissure. The commissures uniting the 



