226 



INNERVATION. 



[CHAP. vni. 



doubtless also in function but its hemispheres are larger masses, 

 of which the interior substance is composed exclusively of fibrous 

 matter, surrounded by a layer of vesicular, which forms a rind or 

 cortex to it. The fibres of the former, however, are prolonged 

 into this cortical layer, and the intermixture of the two forms of 

 nervous substance is thereby effected (fig. 57). 



The spinal cord is composed of certain columns of fibrous sub- 

 stance, in which a large number of the fibres take a longitudinal 

 direction. These, in a great degree, enclose a distinct arrange- 

 ment of vesicular matter, into which, however, as in the cortical 

 layer of the cerebral hemispheres, some, at least, of the fibres of 

 the external white matter are continued, intermingling with its 

 elements. It may, therefore, be stated generally that in the brain 

 the vesicular matter is external and cortical, and in the spinal cord 

 it is internal and almost completely surrounded by the white fibrous 

 matter. This difference of arrangement is probably to be ascribed 

 to the fact, that throughout the whole course of the spinal cord 

 nerves are being given off, whilst from the encephaloii they come 

 only from certain regions. In these regions the white matter is 

 superficial ; but in the hemispheres, from which no nerves proceed, 

 it is deep-seated. We shall describe more minutely the disposi- 

 tion of the two kinds of nervous matter in the cerebro- spinal 

 centre at a future page. 



Of the Nerves and Nervous Centres in Invertebrate Animals. 



Fig. 64. 



Nervous fibres of in:-cct.s : o. Transparent 

 sheath, b. Nerve- fibres, with oval nuclei, c. 

 Shews the bifurcation of the shea tit. 



The description above given applies to 

 the human subject, and to the vertebrate 

 classes generally. In all essential points, 

 so far as the present state of our know- 

 ledge enables us to judge, the structural ar- 

 rangement of the nerves and nervous cen- 

 tres of the invertebrate classes accords 

 with this. Some differences, however, exist 

 which require to be noticed here. In the 

 lobster, the nerve-tubes are large ; the tu- 

 bular membrane has the same transparent, 

 homogeneous appearance, which we have 

 noticed in the vertebrata. But it incloses 

 many delicate nuclei at various intervals. 

 Within the tubular membrane, there is a 

 very thin layer of the white substance of 

 Schwann. The nerve-tubes are very trans- 

 parent, and are much larger than the ave- 

 rage size in vertebrata. Respecting the 

 existence or structure of the gelatinous 



