THE NERVOUS SYSTEM 383 



head, which perceive sounds, and which are instrumental in carrying- 

 on the function of breathing, and as these last are essential to life, 

 destruction of the medulla oblongata is immediately fatal. 



The Pons (c, fig. 174) is a broad and thick band of transverse fibres 

 running across from one hemisphere of the cerebellum to the other. 

 It is traversed by the continuation of the columns of the spinal cord. 

 These emerge from the front border of the pons, and form the diverg- 

 ing crura cerebri (b, fig. 174), the fibres of which radiate outwards to 

 the cortex of the brain. Effusions of blood afiecting these columns cause 

 paralysis. 



Above the pons are the four eminences named the corpora quadri- 

 gemina, which are intimately connected with the function of sight, and 

 are seen in section just above N in fig. 176. 



We now reach the great ganglionic masses situated at the base of 

 the brain, shown in fig. 175. 



These are large, and composed of gray substance, and therefore contain 

 many cells, in which it is probable many of the nerve-fibres in the crura 

 cerebri end, whilst on the other hand they are brought into relation 

 with the cortex of the brain by radiating fibres. The hinder pair are 

 the optic thalami, and are closely connected with the optic tracts. The 

 fi'ont pair are the corpora striata, so termed because the gray substance 

 is traversed by bands of white fibres. These great ganglia are connected 

 by transverse fibres forming the gray and white commissures, and the 

 rest of their opposed surfaces form the lateral boundaries of the third 

 ventricle. 



The Brain. — The large mass of nervous substance which fills the 

 cavity of the cranium or skull, to which the term brain is ordinarily 

 applied, in reality consists of two parts — the cerebrum or brain proper, 

 and the cerebellum or little brain, the proportion of these organs to 

 each other by weight being about as 7:1. Both together weigh in the 

 horse about one pound and a half, which, as compared with the weight 

 of the body, is about as 1 : 600, or about 26 grains for each 1 lb. av. of 

 body weight. 



The Cerebrum, or brain proper, is divided by a deep fi.ssure running 

 from before backwards into two lateral halves. It consists of an immense 

 but thin sheet of gray nerve substance externally, chiefly composed of 

 nerve -cells, and internally of a mass of white substance composed of 

 fibres proceeding from, or running to, the cells of the gray substance. 

 Externally the brain is covered by three protective membranes, as well 

 as by the bones of the skull and skin. These three membranes are the 

 2na mater, which is closely attached to the brain, and is composed 



