712 



THE NERVOUS SYSTEM 



by a triangular band of fibres, the fillet, whieli disappears under the testis and its 

 brachium. 



The crusta, or pes, is a large flattened band of white fibres which is crescentie 

 in section, with the concavity directed backwards and inwards, and in contact with 

 the sul:)stantia nigra. Its antero-inferior, inner, and outer surfaces are free. 

 These surfaces are marked with ridges indicating the bundles of fibres of which it 

 is composed. These ridges do not run parallel to the axis of the crus, but take a 

 slightly curved course, passing at first outwards and then upwards. The crusta is 

 continuous below through the i)ons with the pyramidal ])ody of the same side, and 

 with the cerebellar hemisphere of the opposite side. It is somewhat constricted 

 where it emerges from the pons. It is continuous above with the inner capsule. 



Dissection. — Tlie student sliould scrape away the remains of the lenticular nucleus on the 

 left side until the white substance of the inner capsule is exposed. He should then raise the 

 superficial fibres of the under surface of the crus of the same side, and by tearing off these fibres 

 in an upward direction he may readily demonstrate the continuity of the crusta with the inner 

 capsule. The dissector may also follow the fibres of the crusta downwards to the pyramidal 

 bodies by cutting through the supei-ficial transverse fibres of the pons, but it is better to defer this 

 dissection till a later period. The remainder of the mesencephalon with the optic thalami should 



Fig. 425. — Lateral View of Mesencephalox, Poxs, and Medulla. (Gegenbaur.) 



CORPUS GENICULA TUM 

 EXTERNUM 



CR USTA 



poys 



OLIVARY BODY 



PULVINAR OF OPTIC 

 THALAMUS 



PINEAL BODY 

 CORPUS GENICULATUM 

 INTERNUM 



CORPORA 

 qUADRIGEMINA 



FILLET 



MIDDLE CEREBELLAR 

 PEDUNCLE 



INFERIOR CEREBELLAR 

 PEDUNCLE 



be carefully preserved until the end of the dissection of the brain, when they should be examined 

 in connection with the medulla and i)ons by a series of sections. 



The fibres composing the central third of the crusta are derived from the pyramidal bodies 

 and pass into the anterior half of the posterior linil) of the inner ca])sule to the ascending frontal 

 and ascending parietal convolutions. Its outer third i.s formed by fibres which are i)artly derived 

 from the cerebellar hemispheres of the opposite side, and which pass through the i)Osterior half 

 of the posterior limb of the inner capsule to the temporal and occipital lobes. "J'hese fibres are 

 considered to be senscny in function. Lastly, the fibres constituting its inner third pass upwards 

 through the genu and anterior limb of the cajjsule to the convolutions of the frontal lobe, while 

 below some form communications with the iniclei of the cranial nerves, and others are distributed 

 to the ct'r(!bellum in a similar manner to the fibres of the outer third of the crusta. There are 

 other tracts of minor importance in the crusta. 



The tegmental portions of the crura cerebri, unlike the crusta^, are continuous 

 with one another across the middle line, where they unite in a median raphe. 

 Each tegmen extends into the thalamencephalon beloAv the optic thalamus, wln're 

 it contains a yellowish-grey lenticular body which is called the subthalamic body. 

 The tegmentum contains both grey and white matter, with a continuation of the 

 formatio reticularis of the medulla and ])ons. The most conspicuous of the collec- 

 tions of grey matter is the tegmental or red nucleus, a mass of brownish-grey 



