742 



DISSECTION OF THE BRAIN. 



Composed of 

 three paits. 



Dissection 

 of the cms 

 cerebri. 



of the pons to the under part of the cerebral hemisphere of the same 

 side, near the inner margin. In the natural position the two 

 peduncles occupy the opening in the tentorium cerebelli. Each is 

 about three-quarters of an inch long, and widens as it approaches the 

 cerebrum. Crossing its lower surface is the optic tract ; and between 

 the crura of opposite sides is the interpeduncidar space, which contains 

 the posterior perforated space, the corpora albicantia, and the tuber 

 cinereum with the infundibulum. 



Structure. The peduncle consists of a superficial (lower) layer of 

 white fibres, the crusta, continued from the longitudinal fibres of the 

 pons, a prolongation of the reticulur formation and of other parts 

 termed the tegmentum, and an intermediate stratum of grey matter 

 the substantia nigra. 



Dissection. For the present, the main constituents of the crus 

 cerebri may be made out ; but various accessory parts will be 

 referred to later. If the students are working with two brains, the 

 cut surface of the crura should be examined on the preparation in 

 which the pons and cerebellum have been removed, the fibres of 

 the crusta should be dissected forwards to their entry to the cerebrum 

 and sections should be made of the tegmentum as far forwards as 

 through the anterior corpus quadrigeminum. If only one brain is 

 used the right crus only should be examined. The optic tract 

 should be divided, and the fibres continuous with the pyramid of 

 the medulla oblongata should be raised as far as the junction of the 

 crus with the hemisphere. In this proceeding the substantia nigra 

 (fig. 264, (/) will appear ; and beneath it will be seen the tegmentum. 

 Finally a block of this crus should be removed beneath the 

 quadrigeminal bodies, but leaving them behind, taking care not to 

 transgress the middle line into the left cms. 



The crusta (fig. 269) is composed of coarse bundles of white fibres, 

 ascending from the pons to the cerebral hemisphere, where they enter 

 a layer of white fibres termed the internal capsule, which will be 

 subsequently seen. The continuation of the pyramidal fibres of the 

 medulla oblongata (pyramidal tract) occupies the central part only 

 of the crusta and the lateral parts consist of fibres which have 

 already been traced into the pons. Those on the inner side of the 

 crusta pass from the frontal lobe of the hemisphere, the outer ones 

 from the occipital and temporal lobes, whilst fibres from the fronto- 

 parietal regions, with the pyramidal tract, occupy the intermediate 

 station. 



The substantia nigra (fig. 269) is a layer of dark grey matter which 

 separates the crusta from the tegmentum. In transverse sections it 

 is seen to be convex towards the crusta, and thicker at the inner than 

 at the outer side. 



Tegmentum. The tegmentum is united internally with the like structure of the 

 opposite side below ; but higher up, the two are separated by the 

 grey matter of the posterior perforated space. It consists of a 

 recticular formation continuous with that of the pons, together with 

 a considerable bundle of fibres derived from the cerebellum (superior 



Crusta. 



Substantia 

 nigra. 



