28 DISSECTION OF THE DOG 



the inner fibres are circular in direction. The circular fibres form a somewhat 

 thicker layer in the ileum than in the jejunum. 



Dissection. — Turn to the wall of the thorax, and clean up the scalenus 

 and ventral serratus muscles. 



M. scalenus. — The scalenus muscle cannot be completely exposed at 

 present, since its origin lies in the neck. The part which is to be examined 

 now is applied to the chest-wall dorsal to the transverse muscle of the ribs. 



The scalenus is divisible into three parts, the insertions of which are as 

 follows : (1) The most dorsal is inserted to the cranial border of the third 

 and fourth ribs. (2) The middle part of the muscle is attached to the fifth, 

 sixth, seventh, and eighth ribs. (3) The most ventral part of the muscle 

 is the shortest and reaches the first rib only. It should be noted that a 

 satisfactory differentiation of the dorsal and middle portions of the muscle 

 is frequently impossible. 



Dissection. — Cut across the scalenus muscle on a level with the second rib 

 and remove the part inserted into the ribs. 



M. serrattts VENTRALis. — The ventral serratus muscle is extensive and 

 powerful. Part of it is in the neck and cannot be examined at the present 

 moment. The thoracic portion of the muscle, now exposed, arises by digita- 

 tions from the first seven or eight ribs. The last two or three digitations 

 are related to the origin of the external oblique muscle of the abdomen. 

 The converging bundles of fibres are inserted into the vertebral border 

 and a roughened area on the costal surface of the scapula. 



The nerve supply of the ventral serratus muscle — the long thoracic nerve 

 (n. thoracalis longus) — is derived from the brachial plexus (from seventh and 

 eighth cervical nerves) ; but, instead of accompanying the rest of the branches 

 of the plexus, the nerve runs along the deep face of the scalenus muscle. 



Between the last cervical and the first thoracic digitations of the serratus 

 muscle, the transverse artery of the neck (a. transversa colli) and its companion 

 vein should be noted. The vessels disappear under cover of the serratus. 



Dissection. — Liberate the ventral serratus muscle from all the ribs except 

 the first and turn it outwards. Then clean up the spaces between the 

 ribs as far in a dorsal direction as the attachment of the dorsal serratus 

 muscle, a level generally marked by the appearance of branches from 

 the intercostal vessels. The external oblique abdominal muscle 

 should be detached from the ribs. Observe the extent of the external 

 intercostal muscles. Remove the external muscle from one or two 

 interspaces as far as the attachment of the dorsal serratus muscle, in 

 order to show the underlying internal intercostal muscles. 



