294 THE SPINAL COED. 



the vertebrata, as in the amphioxus, the spinal cord, medulla 



Comparative ' 



nervous system oblongata, and the elementary representatives of the sensory 

 in vertebrates. g an gii a a l ne, and as, in succession, we pass to the higher 

 ones, we recognize a cerebellum appearing over the medulla oblongata, 

 and cerebral hemispheres over the sensory ganglia. These organs in 

 the upward career become more and more developed, the hemispheres, for 

 example, soon equaling in size the quadrigemina, and then greatly sur- 

 passing them, and with this increase of size a higher grade of intelligence 

 is reached. In fishes there are four ganglia, corresponding respectively 

 to the cerebellum, quadrigemina, cerebral hemispheres, and olfactive gan- 

 glia. In reptiles the number of ganglia and their order of occurrence is 

 the same, but the cerebral hemispheres have now greatly increased, an 

 increase which is even better marked in birds, for in them the hem- 

 ispheres have expanded in front so as to cover the olfactive ganglia, and 

 posteriorly the optic, a condition of things analogous to that presented 

 by the human brain at about the close of the third month of foatal life, 

 and approaching that permanently exhibited by the lower mammals, as, 

 for instance, the marsupials. It is to be understood that what is here 

 spoken of as the hemispheres answers in reality only to the anterior lobe 

 of the cerebrum of man ; and as in him, during the fourth and fifth months, 

 the middle lobes are developed in the upward and backward direction 

 from the anterior, and still later the posterior lobes from the posterior of 

 these, the same course is followed in the animal series, the final type of 

 development, the trilobed cerebrum, being only reached by the highest 

 carnivora and quadrumanous animals. 



Commencing now more particularly with human nervous anatomy 



STRUCTURE OF THE SPINAL CORD. 



The spinal cord is placed in the midst of the vertebral canal. In form 

 Description of it is cylmdroid, its section being elliptical, the lateral diame- 

 the spinal cord. ter teing the long on ^ Longitudinally it shows two en- 

 largements, one about its upper third, the other toward its termination. 

 Exteriorly it is white, but its section shows a gray substance, arranged 

 in the form of two crescents connected by an isthmus. Above, it is con- 

 tinuous with the brain, which, indeed, is a development upon it, and be- 

 low it terminates at the cauda equina. Its relative length is much great- 

 er in foetal life, at the third month of which it extends into the sacrum. 

 In adult life it only occupies about the upper two thirds of the verte- 

 bral canal ; it is generally stated that its termination is about the first 

 or second lumbar vertebra. Moreover, it does not fill the vertebral ca- 

 nal, being, by reason of the transverse dimensions of that cavity, rather 

 suspended in than confined by it. The rest of the space, amount- 

 ing to about one third, is occupied by the roots 'of the nerves, liga- 



