THE NERVES. 803 



It may be added tl>at ti.e olfactory lobes arise, a« i„ auhu-A.. f,o,n two „r.l. r. of roots, but 

 they are small a.i.l entirely hidden beneata the inferior facr ot the fronial lobes 



Lxaimne.1 internally, the brain of Man offers the folluwin- principal differences :- 

 1 he corpus callosum is very developed, and, above tlie ventricle, forms, Iro.n before to 

 behind, a salient angular prolonj^ation named the frontal cor„u and occipital prolongation, 

 or jorceps major. r r y t 



There is nothinf? to note concerning the fornix, and septum lucidum, except that there is 

 thHi/ia "" ^ com.uunicates with the middle ventricle by a small aperture- 



The lateral ventricles offer remarkable differences. They are not prolonged into the 

 olfactory lobes, but possess a diverticulum that enters the occipital lobe, below the forcepa 

 n.ajor. This space .8 more or less developed, and terminates in a point; it is named the 

 oncyvoid or digital cavity, and shows on its floor a small convolution which has been designated 

 the ergot of Mar and (pes hippocampi). The divertieuluni and convolution do not exist in 

 animals. Ihe cornu Ammouis is slightly uneven on its surface; it is limit..d, inwardly, by 

 a b;md, and below this by a grey denticulated lamina-the g>,rus fornicatus. 



The optic thalamus forms an enormous prominence on the rt.<.r of the lateral ventricle, 

 between the caudate nucleus and the cornu Ammonis, while it is entirely covered by the latter 

 in the brain of animals. 



THIRD SECTION. 

 THE NERVES, 



The mrves represent the peripheral portions of the apparatus of innervation 

 They are cords ramifying in every part of the body, having their origin in the 

 spinal cord or its prolongation— the brain. Before commencing their special 

 study It IS necessary to possess a summary notion of the principal distinctions 

 of which they are susceptible, with regard to their origin, distribution and 

 termination. 



Structure.— The nerves are formed by an aggregation of the nerve-tubes 

 already described. These are grouped in primary fasciculi, which are rectilinear 

 or slightly undulating, and ei.\eloped in a sheath of delicate connective tissue— 

 the pefmeunion (intemum). These primary fasciculi are again collected into 

 bundles to form secondary fasciculi, which are maintained by a layer of fibrillar 

 connective tissue thicker than the perineurium {perinewium externum). Finally 

 these secondary bundles by their union constitute the nerve, around which the 

 connective tissue becomes condensed, and constitutes the neurilemma. 



(Extremely small septa pass into the secondary bundles of nerves, consti- 

 tuting the emloneurium ; from this delicate folds or lamella are given off around 

 each nerve-fibre, corresponding to the perimysium of individual muscle-fibres. 

 1 he coverings of the nerves are anatomically continuous with the connective 

 tissue coverings of the brain and spinal cord. The perineurium consists both of 

 (.rdiiiary connective tissue and elastic tissue ; and it is lined by two, and in 

 small nerves by one, layer of endothelial cells, forming the ^henth of Henle ) 



I essels traverse the connective tissue separating the fasciculi from each 

 other : they anastomose in a network with elongated meshes which are parallel 

 with the nerve-tubes, and they are also surrounded by the nervC nervorum. 



(Lymph-spaces exist between the layers or lamelU« of the perineurium and it 

 would also appear that each individual nerve is surrounded by a lymphatic 

 space, which is continuous with the subdural and subarachnoid lymph-spaces 



