118 MESENCEPHALON. 



that although in many mammals (mouse, guinea-pig), and in all vertebrates below 

 mammals, the decussation is complete, so that all the fibres pass from one tract 

 into the opposite optic nerve, in other mammals (rabbit, dog, cat, monkey) and 

 in man, it is incomplete, so that some of the fibres from the left optic nerve pass 

 into the left tract and others into the right tract, and the same mutatis mutandis 

 for those of the right nerve. This fact has been conclusively proved both by 

 experiment and clinical observation. At the same time it must be stated that cases 

 have occasionally been recorded which, if correct, are only to be explained on the 

 supposition that the crossing is complete : in these we must suppose that there has 

 been a reversion to the lower and more primitive type of chiasma.. 



The two sets of nerve-fibres are usually spoken of as the crossed and uncrossed 

 bundles, although they are not to be regarded as formed into entirely separate 

 funiculi, nor are their limits sharply defined either in the nerve, chiasma, or tract. 

 The uncrossed bundle is distributed exactly to the lateral half of the retina, the 

 crossed bundle to the mesial half ; and on the whole the fibres of the uncrossed 

 bundle keep to the lateral side of the optic nerve, whilst the crossed bundle is on 

 the mesial side of the nerve. In the chiasma and tract they are usually more 

 commingled, and their relative position is very variously described. In a case of 

 atrophy of one nerve which has been clearly described by Williamson, the uncrossed 

 fibres chiefly occupied the middle of the tract, the crossed being chiefly at the 

 periphery and lower part. Similar results were obtained by Henschen. 



According to Salzer's enumeration, there are nearly half a million fibres in each optic nerve. 

 The fibres vary in size. According to v. Gudden they are divisible, according to their calibre, 

 into two classes, finer and coarser. The former of these he regarded as being concerned in 

 the production, as afferent fibres, of the pupil-reflexes, the latter as conveying visual impressions. 

 The finer fibres degenerate after destruction of the anterior corpora quadrigemina (in the cat 

 and rabbit). It must be remembered that not only afferent, but also efferent impressions are 

 carried by the optic tracts ; governing the chemical changes which occur in the retina, and 

 also the movements of some of its elements (pigment-cells, cones). 



Besides the optic nerve-fibres, there is a bundle at the posterior part of the 

 chiasma, and running along the mesial side of the optic tracts to join the internal 

 geniculate bodies of the two sides. 1 This is known as the inferior commissure or 

 commissure of Gudden. It appears to have no relation to the visual function, for it 

 undergoes no change when the rest of the optic tracts become atrophied after extir- 

 pation of both eyes in young animals. Its fibres are of extremely small diameter. 



In structure the optic tract resembles an extension of the white matter of the 

 brain, being very soft and not divided up into bundles by connective tissue, nor are 

 its fibres provided with membranous sheaths. These structures are found, however, 

 in the chiasma, and in the optic nerve, which are strong and tough and invested 

 with sheaths continuous with the pia mater and arachnoid. At its entrance into 

 the orbit the optic nerve receives a strong investment from the dura mater, which 

 is continued around it into the sclerotic coat of the eye. The nerve is subdivided 

 by intercommunicating connective-tissue septa into a large number of compartments, 

 which contain the nerve-fibres, but the latter are not gathered up into round funiculi 

 invested by perineurium, as in .the case of the ordinary peripheral nerves. 



Stilling has described the optic tract as sending a prolongation directly into the outer part 

 of the crus cerebri. This prolongation is said to contain fibres which pass (1) to the oculo- 

 motor nuclei ; (2) to the cerebellum by the inferior peduncle : (3) to the pons Varolii, even 

 extending to the inferior olive. 



The name traetus transrersus pcdvnculi was given by Gudden to a band of fibres, first 

 noticed by Inzani and Lemoigne, which is sometimes seen emerging from the base of the 

 anterior quadrigeminal body, and coursing obliquely over the brachia of the quadrigeminal 

 bodies to enter the crus cerebri. It is much better marked in some animals than in man. 

 It becomes atrophied after extirpation of the eye, and may be a part of the prolongation 

 of the optic tract described by Stilling. 



1 It is stated by Darkschewitsch, that this commissure unites each mesial geniculate body with 

 the lenticular nucleus of the opposite side. 



