652 THE BRAIN. 



we must also probably add the pons. We may exclude the pes of the crus, 

 since this is composed exclusively of fibres,bringing the cerebral hemispheres, 

 including the corpora striata, into connection with the pons, bulb, and cord, 

 and so with the coordinating machinery itself, as well as with other parts of 

 the nervous system. And observation, as far as its goes, supports this deduc- 

 tion from anatomical relationships. We will, however, defer what else we 

 have to say on this point until after we have discussed the carrying out of 

 voluntary movements. 



ON SOME HISTOLOGICAL FEATURES OF THE BRAIN. 



560. The white matter of the brain, as we have already said, like that 

 of the spinal cord, consists of medullated fibres of various sizes imbedded 

 in neuroglia and supported by septa of connective tissue derived from the pia 

 mater. Save that cells, or even groups or rows of cells, for the most part 

 small cells, about many of which it may be debated whether they are nerve- 

 cells or neuroglia cells, arejfrequently seen between the fibres and bundles of 

 fibres, the white matter of the brain seems essentially identical with that of 

 the spinal cord. 



The gray matter of the brain in general also corresponds to the gray 

 matter of the cord in consisting of branching nerve-cells, fine medullated 

 fibres of peculiar nature, non-medullated fibres and fibrils, with a few ordi- 

 nary medullated fibres, all supported in neuroglia. 



The " central " gray matter is extremely like that of the cord except 

 that the nervous elements are imbedded in a relatively larger quantity of 

 neuroglia. Immediately underneath the epithelium lining the several ven- 

 tricles and the aqueduct, the neuroglia is especially developed, forming a 

 distinct layer which may be regarded as a continuation of the central gelat- 

 inous substance of the spinal cord, and which, with the epithelium overlying 

 it, forms what is known as the ependyma. The " nuclei " of the cranial 

 nerves are, as we have seen, comparable to the groups of nerve-cells in the 

 spinal cord. 



A great deal of gray matter of the brain may be spoken of as more 

 " diffuse " or " scattered," more broken up by bundles of fibres than is the 

 case in the spinal cord. The " reticular formation " of the bulb and of the 

 tegmental region is an extreme form of this diffuse gray matter. And even 

 in such collections of indubitable gray matter as the corpus striatum, optic 

 thalamus, and the like, the pure gray matter, if we may use the term, is 

 much more interrupted and broken up by conspicuous bundles of white 

 fibres than is the case in any region of the spinal cord. In the corpora 

 quadrigemina the gray matter is broken up by sheets or bundles of white 

 matter. 



The nerve-cells of the several collections of gray matter are not all alike ; 

 they present in different regions differences in size, form, and in other cha- 

 racters. The cells of the nucleus caudatus, for instance, are rather small and 

 often round or spindle-shaped, while those of the optic thalamus are large, 

 branched, and rich in pigment. The cells of the substantia nigra are spindle- 

 shaped, of moderate size, and so loaded with black pigment (in man) as to 

 justify the name ; those of the locus cseruleus are very large and spherical, 

 with just so much pigment as to give a bluish tint. But our knowledge of 

 the finer histological details of the various masses of gray matter is at present 

 too imperfect to afford any basis whatever for physiological deductions; and 

 it will be hardly profitable to dwell upon them. Two regions of gray matter 

 alone call for special description, the cortex cerebri and the superficial gray 

 matter of the cerebellum. 



