442 THE ANATOMY OF THE HORSE 



1st. The umbilicus, which leaves a weak place in the abdominal parietes, 

 especially at and soon after birth, through which umbilical rupture takes 

 place. 2nd. The external ring, and the canal of which it is the outer 

 boundary, together with the internal ring, which should be carefully 

 examined by the student of veterinary surgery. Want of space will, 

 however, forbid more than a general description here. The external ring 

 has already been described as formed by the posterior tendinous fibres of 

 the external oblique, and through this descends the spermatic cord to the 

 scrotum. In tracing backwards and outwards this cord to the point whera 

 it enters the wall of the abdomen, it will be found to lie between the fibres 

 of the crural arch and those of the internal oblique, supported by the 

 peritoneum and a thin fascia, which is continued from the edge of the 

 transversalis muscle, but is not so distinct as the corresponding part in the 

 human subject. About three or four inches from the external ring this 

 fascia is pierced by the cord, and this part is called the internal abdominal 

 ring, the space between the two rings being the inguinal canal. (See 

 Spermatic Cord, Chapter xxii.) 



DEEP ABDOMINAL REGION 



The abdomen is bounded by a muscular wall anteriorly, which forms a 

 movable septum between it and the chest, and is called the diaphragm. 

 Superiorly also there are the muscles which serve to bend the spine down- 

 wards, in opposition to the dorsal muscles. 



The diaphragm consists of a large flat muscle and two crura, with a thin 

 circular layer of tendon in the centre. The former arises by fleshy digitations 

 from the cartilages of the ribs, from the eighth to the sixteenth inclusive, 

 and from the ensiform cartilage. It is inserted into a central flat tendon of 

 a circular shape. Each CRUS arises from its corresponding side of the bodies 

 of the lumbar vertebrte : the two cross each other opposite the seventeenth 

 dorsal vertebra, and again decussate after allowing the oesophagus to pass 

 through, being finally attached to the central tendon. Between the crura 

 and the bodies of tlie vertebrae the aorta passes backwards, and in the 

 central tendon is the opening for the vena cava posterioi\ The action of this 

 muscle is to diminish the capacity of the thorax by reducing the convexity 

 of its surface. 



SeMI-SPINALIS LUMBORUM, INTERTRANSVERSALIS LUMBORUM, and SACRO 



LUMBALis, are three muscles having numerous attachments to the transverse 

 processes of the posterior dorsal and lumbar vertebrpe and sacrum. Their 

 action is to approximate the pelvis to the thorax, and thus to oppose the 

 dorsal muscles by rounding the back. 



Psoas magnus is a long and strong muscle lying beneath the spine. Its 

 ori'jin is from the necks of the last two ribs, and from the bodies and 

 transverse processes of the last dorsal and all the lumbar vertebra). Insertion 

 — into the trochanter minor internus of the femur. Action — to flex the 

 haunch upon the pelvis, or, if the hind-leg is fixed, to assist the three last 

 muscles in rounding the back. 



Psoas pakvus lies along the inner side of the P. magnus. Oriyin — from 



