MEDULLA OBLONGATA. 473 



the inferior roots of the nerves. The connective tissue of the cord 

 is a reticular variety, very delicate and gelatinous in its structure ; 

 it is termed the neuroglia. In the middle of the grey commis- 

 sure the central spinal canal runs the whole length of the cord, 

 being continuous anteriorly with the fourth ventricle of the 

 brain. 



The fibres of the cord are not all continuous with those of the 

 encephalon, some being proper to the cord itself; while of those 

 which proceed to the brain, some decussate with each other before 

 doing so. 



The proportionate size of the brain and spinal cord varies in 

 different animals, according to the position which they occupy in 

 the scale of intelligence ; the higher that position the larger and 

 heavier proportionately is the brain, and the smaller and lighter the 

 cord relatively. Thus in man, the most intelligent of the animal 

 creation, the brain averages 50 oz., and the spinal cord about 1^ 

 oz. — a proportionate weight of 1 to 33. In the horse the brain 

 averages about 23 oz., and the spinal cord 10| oz,, or a propor- 

 tionate weight of 1 to 2*19. 



THE ENCEPHALON, 



The encephalon or brain is that portion of the cerebro-spinal 

 axis situated within the cranium. In form it is a slightly flattened 

 and elongated ovoid body, which may be considered as consisting 

 of four parts — the medulla oblongata, the pons VaroUi, the 

 cerebellum, and the cerebrum. 



MEDULLA OBLONGATA. 



This is the prolongation of the spinal cord, extending to the 

 pons Varolii, from which it is separated by a shallow fissure. 

 This portion of the brain is very large in the domesticated 

 animals, and wherever the spinal cord is largely developed It 

 is pyramidal in shape, the narrowest part being continuous with, 

 the cord. The inferior surface is convex, and rests upon the 

 basilar process, and the superior surface is received between the 

 hemispheres of the cerebellum, where it forms the floor of the 

 fourth ventricle. Two median fissures, continuous with the supe- 

 rior and inferior longitudinal fissures of the cord, divide the medulla 

 into symmetrical halves. The superior, deep and narrow, expands 



