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 com- 

 missure 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 proportionate 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 

 Varolii, the cerehellum, 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 

 lateral lobes of the cerebellum, where it forms the floor of the 

 fourth ventricle. Two median fissures, continuous with the 

 superior and inferior longitudinal fissures of the cord, divide the 

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



