THE NERVES. 803 



It may be added that the olfactory lobes arise, as in animals, from two orders of roots, but 

 they are small and entirely hidden beneath the inferior face of the frontal lobes. 



Examined internally, the brain of Man offers the following principal differences :— 



The corpus callosum is very developed, and, above the ventricle, forms, from before to 

 behind, a salient angular prolongation named the frontal cornu and occipital prolongation, 

 ov forceps major. 



There is nothing to note concerning the fornix and septum lucidum, except that there is 

 a ventricle in the latter which communicates with the middle ventricle by a small aperture — 

 the vulva. 



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 forceps 

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

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

 the ergot of Morand (pes hippocampi). The diverticulum and convolution do not exist in 

 animals. The cornu Ammonis is slightly uneven on its surface ; it is limited, inwardly, by 

 a band, and belnw this by a grey denticulated lamina— the gyrus fornicatus. 



The optic thalamus forms an enormous prominence on the floor of the lateral ventricle, 

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

 in the brain of animals. 



THIRD SECTION. 

 THE NERVES. 



The n&rves 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 enveloped in a sheath of delicate connective tissue — • 

 the perineurium {internum). 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 {perineurium 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 endoneurium ; from this delicate folds or lamellae are given off around 

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

 The coverings of the nerves are anatomically continuous with the connective 

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

 ordinary connective tissue and elastic tissue ; and it is hned by two, and in 

 small nerves by one, layer of endothelial cells, forming the sheath of Henle.) 



Vessels 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 tiervi nervorum. 



(Lymph-spaces exist between the layers or lamellse 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 



