140 THE HORSE 



The substance of the brain consists of white and grey 

 matter, the grey matter being disposed externally, 

 whereas the reverse is applicable to the spinal cord, in 

 which the grey matter is placed internally. 'Like the brain, 

 the cord has three coverings. 



Highly specialised nerve cells enter largely into the 

 formation of the structure of the brain and the spinal 

 cord. Twenty-two pairs of nerves arise from the cord, 

 conveying afferent and efferent impulses to and from it. 



The Respiratory System 



In a horse the respiratory system (or respiratory organs) 

 is well developed, and comprises the nasal cavity, the air 

 sinuses, the larynx, the trachea, and the lungs, plus the 

 bronchial tubes. The nostrils are fairly wide and divisible 

 into two portions, one being known as the false nostril, 

 and one as the true. The nasal passage is divisible into 

 right and left portions, the partition between the two 

 being composed of a plate of cartilage known as the 

 septum nasi, and this, like the remainder of the passage 

 as well as the air sinuses, is covered by mucous membrane, 

 which in health is of a bright pink colour. 



The air sinuses are compartments formed within the 

 skull, and contain a reserve supply of air. They are in 

 communication with one another and with the nasal 

 passage, and in addition confer a degree of lightness on 

 the head. 



The horse, being an animal required for swift pro- 

 gression, renders it necessary that the foregoing features 

 should be present. The sinuses are divided into the 

 superior maxillary, the inferior maxillary, the frontal, 

 and the sphenoidal, the last named being the smallest of 

 the four. The sinuses are not uncommonly implicated in 

 various diseases. 



The larynx forms the entrance to the trachea or great 

 air tube, and it is particularly well developed in the 

 horse. It is composed of cartilage, muscles, and inter- 



