102 THE ANATOMY OF THE HORSE. 



to the corresponding branches of the dorsal nerves in the back. They 

 supply muscular branches to the muscles over the lumbar transverse 

 processes, and cutaneous twigs to the skin of the loins and croup. 

 Branches of the lumbar arteries and veins accompany these nerves. 

 Each artery sends a spinal branch through the intervertebral fora- 

 men. 



THE CAVITY OF THE THORAX. 



Directions. — In order to expose the thoracic cavity, the chest walls 

 must be in part removed ; and it is most convenient, in the first place, 

 to make the opening on the left side. The trunk should be allowed to 

 remain in the suspended position. If the diaphragm is intact, and if no 

 opening quite through the chest-wall has been made in the previous 

 dissection, then the first step should be to perforate one of the intercostal 

 spaces with the finger or a blunt instrument. This is to be done in 

 order to allow the lungs to collapse ; and a sharp instrument must not 

 be used, lest the surface of the lung might be injured. As soon as the 

 finger or instrument is withdrawn from the aperture, the air will be 

 heard to rush in and fill the pleural cavity, which was previously 

 occupied by the distended lung. This is precisely what occurs when 

 the chest-wall is perforated in the living animal, in which, in health, the 

 outer surface of the lung is closely applied to the inner surface of the 

 wall, following it in all its movements. The lung is kept in this dis- 

 tended state by the atmospheric pressure, which operates on the air 

 passages in the interior of the lung, but not on its exterior, where the 

 pressure is borne by the chest wall ; and the lung is kept thus distended 

 in opposition to a strong natural tendency to contract, which it possesses 

 in virtue of the large amount of elastic tissue in its structure. But 

 when the wall of the chest is perforated the pressure of the atmosphere 

 becomes exerted on the exterior as well as the interior of the lung, and 

 the unopposed elasticity of the lung texture then comes into play. 



By means of the saw and bone forceps, the ribs, except the first and 

 a few at the end of the series, are to be removed, the upper section 

 being made a few inches below the head of each rib, and the lower a 

 little above the chondro-costal articulation. 



Form and Boundaries of the Cavity (Plates 22 and 25). — If the con- 

 tained organs were removed from the thorax, and a cast were taken of its 

 interior, it would be found to have an irregularly conical form, but the 

 symmetry of the cone is largely departed from. The base of the cone 

 is formed by the diaphragm, which, viewed from the thoracic side, is 

 markedly convex, like the roof of a dome. The plane of attachment 

 of the diaphragm slopes downwards and forwards, so that the antero- 

 posterior measurement of the cavity is much less below than above. 

 Moreover, as the diaphragm is dome-shaped, this measurement is less 



