OUTLINE OF THE STRUCTURE OF THE HORSE. 55 



tern of ganglia, which specially controls the vital organs of 

 circulation, digestion, and excretion. The first five divisions 

 are generally included under the head of the nervous system 

 of animal life, the last being considered to be peculiar to 

 organic life." 



Before passing to the next section, we must not forget to 

 mention the membranes of the brain. These are three in 

 number, each completely investing it. The outer or upper 

 one is called the dura mater, and is fibrous in texture, and quite 

 strong. The middle one, called arachnoid, is a serous mem- 

 ber of the ordinary character. The inner one, denominated 

 the pia mater, is full of vessels, and is by far the most tender. 

 Being next the brain, however, it is less exposed to injury 

 than the others. The pia mater penetrates into every depres- 

 sion, lines every ventricle, and clothes every portion of the 

 brain. 



RESPIRATORY ORGANS. 



The important function of respiration is carried on through 

 the lungs and the nasal cavities, and the tubes by which these 

 are brought into communication, with the assistance of the 

 inspiratory muscles and the system of respiratory nerves. 



The lungs are invested by the pleura, a serous membrane,, 

 which is reflected or doubled back upon the walls of the 

 thorax, or chest. They consist of two conical, spongy bodies, 

 called the right and left lobes of the lungs. They are di- 

 vided from each other by a doubling of the pleura and a 

 space, called the mediastinum., which is occupied by the heart, 

 great blood-vessels, nerves, and glands. The right lobe is 

 noticeably the larger, by which arrangement the lungs are 

 better adapted to the form of the chest, and enabled more 

 perfectly to fill it. In consequence of their extremely cellu* 

 lar or porous structure, they are capable of great expansion 

 and contraction during the operation of breathing. 



The substance of which the lungs are mainly composed is 

 the pulmonary tissue, termed the parenchyma. This, in the 

 healthful subject, has a beautiful pale-rose color. It is very 



