462 THE NEKVOUS SYSTEM. 



motor branches are those (x, x, x) going to the temporal, niasseter, 

 and two pterygoid muscles, and that distributed (y) to the mylohyoid 

 muscle and the anterior belly of the digastric. 



The remaining portion of the trigeminus then enters the dental canal 

 of the inferior maxilla, through which it passes, giving off filaments to 

 the teeth and gums of the lower jaw. It finally .emerges at the mental 

 foramen, and is distributed in numerous diverging ramifications to the 

 integument of the chin and edge of the under jaw, and the skin and 

 mucous membrane of the lower lip. 



Physiological Properties of the Fifth Pair. The most prominent 

 character of this nerve is its general sensibility. The regions to which 

 it is distributed, namely, the cheeks, eyelids, tip of the nose, lips, an- 

 terior nares, and especially the tip of the tongue, possess a tactile sen- 

 sibility of higher grade than most other parts of the body. The nerve 

 itself, together with its principal branches, is acutely sensitive to me- 

 chanical irritation, and will give rise to indications of sensibility under 

 conditions when the spinal nerves are nearly or quite inactive. 



But the most direct and conclusive proof of the function of this 

 nerve is the loss of sensibility produced by its division. If either 

 the infraorbital or the mental branch be divided at its exit from the 

 superior or inferior maxilla, tactile sensibility is impaired or abolished 

 in the corresponding region of the face. A still more striking result 

 is produced by dividing the entire nerve within the cranium. This 

 operation, which was first performed by Magendie, may be done, upon 

 the cat or the rabbit, by means of a steel instrument with a slender 

 shank and a narrow cutting blade projecting at nearly a right angle 

 from its extremity. The instrument is introduced in a horizontal direc- 

 tion through the squamous portion of the temporal bone, and pushed 

 inward and forward, with its blade lying flatwise on the floor of the 

 skull, until it strikes the posterior clinoid process. It is then slightly 

 withdrawn, its cutting edge turned downward, and the nerve divided 

 where it crosses the petrous portion of the temporal bone. By this 

 method all its fibres are cut off, and the only part of the brain neces- 

 sarily wounded is the inferior portion of the temporal lobe. 



The immediate effect of this operation is a complete anassthesia of 

 the integument and mucous membranes about the face on the operated 

 side. The cornea can be touched without exciting any movement of 

 the eyelids. A probe may be introduced into the nasal passages, or the 

 lips may be pierced with a needle, without eliciting any sign of sensa- 

 tion on the part of the animal. At the same time the power of motion 

 in these parts is unaffected. The eyelids may be opened or closed under 

 the influence of visual impressions, and the movements of the lips con- 

 tinue to be performed in a nearly natural manner. In the cat, the loss 

 of sensibility and persistence of the power of motion is shown by irri- 

 tating at different points the integument of the external ear, which in 

 this animal has an acute tactile sensibility. If a pointed instrument 

 be brought in contact, on the operated side, with the anterior part of 



