216 THE HEART. 



leap with empty lungs. The inspiration is full, being neces- 

 sary, not only to fill a large cavity with gas, and thus 

 lighten the body, but to render the ribs fixed ; so enabhng 

 the muscles connected with them to act with more enere;y. 

 In this the gluteus maximus operates {Plate II. in). The 

 muscles proper to the thigh contract, and by this means 

 render firm the pelvis, though at the same time the hind- 

 most part of the sacrum is drawn slightly downward. 

 Upon these fixed points the longissimus dorsi {Plate III. F), 

 the spinahs dorsi {Plate III. G), the superficialis costa- 

 rum {Plate III. b), the transversalis costarum, &c. contract ; 

 and acting together with the gluteus maximus, in the first 

 plac^e draw close the upper part of the ribs and the spines 

 of the dorsal vertebr?e, rendering the horse hollow backed ; 

 then raise the trunk upon the natural hinge of the ver- 

 tebrse, or the back upon the sacrum (see Skel.) ; between 

 the which there is an obvious space, and the spines of 

 which abruptly incline different ways. The whole mass of 

 contractile power now comes into play ; the forward mus- 

 cles of the back drawing up the neck at the same time, and 

 the body is raised. The hinder feet then give a spring, 

 and the leap is made ; after which the muscles thus vio- 

 lently used as suddenly relax, and expiration takes place. 



The nerves which supply the pleurae are derived from 

 the intercostals and phrenic, the par vagum and sympa- 

 thetic. The pleurae have no sensation when in health, but 

 like most parts w^e are unconscious of when they are well, 

 they become acutely sensitive in disease. The aiteries are 

 very small, and are given off" from any of the neighbouring 

 trunks ; and the veins, which are also extremely minute, • 

 empty themselves into the adjacent veins. 



THE HEART. 



The heart is covered by a double fold of a serous sac, 

 which secretes about an ounce of clear liquid. Like all 

 serous membranes it is closed, touching nothing but its 

 secretion or itself, being favourable to motion. To un- 

 derstand this, let us again suppose we hold in our hand 

 a soft, flexible bladder, twice as large as the heart itself, 

 w^hich has no opening ; we then take the heart, and for- 

 cibly indent the bladder with it. The bladder adheres to 



