370 



FOURTH PAIR OF NERVES. 



water radii brevis, to its inner the extensor primi 

 internodii pollicis. It is covered posteriorly by 

 the extensor communis digitorum and extensor 

 carpi ulnaris. Gliding downwards and out- 

 wards from beneath these muscles and becoming 

 tendinous on its under surface, it slips over the 

 lower third of the posterior face of the radius, 

 and then running in a groove on the outer side 

 of that bone, common to it and the next men- 

 tioned muscle, is ultimately inserted into the 

 metacarpal bone of the thumb. 



The action of this muscle is to extend the 

 metacarpal bone of the thumb, which corre- 

 sponds as regards its capacity for motion, with 

 the phalanges of the fingers. 



3. Extensor primi internodii (extensor 

 minor pollicis manus, Soemm.; cubito sus- 

 phalangettien du pouce, Chauss.) is a very small 

 muscle compared with the last, though varying 

 much in size in different subjects. It lies 

 between the extensor ossis metacarpi pollicis 

 and the extensor secundi internodii, passing 

 through the same groove in the radius as the 

 extensor ossis metacarpi pollicis, it becomes 

 inserted into the first phalanx of the thumb. 



4. The extensor secundi internodii pollicis 

 (extensor major pollicis manus, Soemm. ; cubito 

 sus-phalangettien du pouce, Chauss.) This 

 muscle is usually larger than the former ; it 

 arises fleshy from the ulna and interosseous 

 ligament ; becoming tendinous in its centre, it 

 presents the same penniform appearance referred 

 to above. The groove in the radius which is 

 narrow and deep, this tendon alone being 

 lodged in it. It is situated between that for 

 the two radial extensors and the broad and 

 hollow one for the common extensors and 

 indicator. Crossing on the back of the wrist 

 the radial extensors it is finally inserted into 

 the second phalanx of the thumb. This mus- 

 cle is entirely covered in the fore-arm by the 

 common extensors of the fingers. 



5. The indicator (cubito sus-phalangettien 

 de I'index) occupies the remaining portion of 

 the posterior interosseal space. Like the three 

 last described muscles it is penniform, and 

 tirises fleshy from the ulna and interosseous 

 ligament, it descends to the hand and passes 

 through the same groove at the back of the 

 radius as that of the extensor communis digi- 

 torum, it is inserted into the posterior surface 

 of the three phalanges of the index finger. 

 This muscle is entirely concealed by the exten- 

 sor carpi ulnaris and extensor communis digi- 

 torum. 



The names of this and the two preceding 

 muscles indicate their actions. 



For BIBLIOGRAPHY, see ANATOMY, (INTRO- 

 DUCTION.) 



(Samuel Solly.) 



FOURTH PAIR OF NERVES (nervus 

 trochlearis, s. patheticus). The fourth pair is 

 the most slender of the encephalic nerves. They 

 are intermediate in the order of succession to 

 the third or motor oculi and the fifth nerves, and 

 hence the name. Each nerve is attached at its 

 encephalic extremity to the lateral part of the 



superior surface of the anterior medullary 

 velum or valve of Vieussens, immediately behind 

 the testes or the posterior of the tubercula 

 quadrigemina. It is divided at its attachment, 

 for the most part, into two roots, inserted at a 

 little distance from each other, one close to the 

 testes, the other posterior to it. Occasionally it 

 has but one root and sometimes even three. 

 Gall and Spurzheim* are of opinion that the 

 nerve might be traced to a more remote point, 

 and according to May of " its fibrils appear to 

 pass through the filaments of the pillar of the 

 valve, and in part to arise from the back part 

 of the medulla oblongata." 



The nerve is concealed at its insertion by the 

 superior vermiform process of the cerebellum, 

 and it is not immediately provided with neu- 

 rilemma, and hence, as also because of its 

 delicacy, it is very easily broken off. 



Its course within the cranium is circuitous 

 and long, longer than that of the other nerves. 

 It passes outward, downward, and forward : 

 it first descends external to the superior peduncle 

 of the cerebellum (the processus a cerebello ad 

 testes), between it and the vermiform process, 

 then becomes invested with arachnoid mem- 

 brane and free, and runs round the lateral part 

 of the crus cerebri, above the superior margin 

 of the pons Varolii, and beneath the free edge 

 of the tentorium cerebelli, until it reaches the 

 posterior clinoid process of the sphenoid bone ; 

 it then enters the outer wall of the cavernous 

 sinus between the points of attachment of the 

 tentorium, and is transmitted through a canal 

 in the dura mater, by which the wall is formed, 

 forward to the foramen lacerum of the orbit. 

 It does not enter the sinus, being contained in 

 a canal in its outer wall. 



At the posterior part of the sinus the nerve 

 is situate beneath the third, between it and the 

 first division of the fifth nerve; but at the 

 anterior, and as they are about to pass into the 

 orbit, the fourth and frontal branch of the fifth 

 are both above the third, the fourth internal 

 and a little superior to the frontal. 



The nerve lastly enters the orbit through the 

 superior part of the foramen lacerum in com- 

 pany with the frontal, above the levator palpebne 

 muscle, and immediately beneath the roof of 

 the region. Having entered, it runs forward 

 and inward, gains the surface of the superior 

 oblique muscle, and attaching itself to it upon 

 its superior aspect, about its middle, it divides 

 into filaments, which are all distributed to the 

 muscle. 



The fourth nerve does not give off any branch 

 during its course to the oblique muscle, unless, 

 at times, first a filament described by Cruveilhier, 

 and, according to him,distributed to the tentorium 

 cerebelli. This filament arises from the nerve 

 while traversing the wall of the cavernous sinus, 

 runs backward into the substance of the ten- 

 torium, and divides into two or three branches: 

 Cruveilhier calls it " the branch of the tento- 

 rium." Secondly, according to both Swan and 

 Cruveilhier, the fourth nerve gives off a fila- 



* Anatomic du Systeme nerveux. 

 t Physiology. 



