9 DISSECTION OF THE BACK AND TIJORAX. 97 



The Serratus Magnus (Plate 4). This muscle will be seen here, as 

 left by the dissector of the fore limb. The student should notice its 

 mode of origin from the ribs (see page 7), and then carefully remove it. 



Directions. — The' latissimus dorsi must now be removed, beginning 

 below, where its muscular portion was cut by the dissector of the fore 

 limb. This operation must be conducted with' care, in order to leave 

 intact the anterior and posterior serratus muscles, whose thin tendons 

 are adherent to that of the latissimus. Indeed, over the last ribs, in 

 an old subject, it will be found impossible to separate the latissimus 

 from the underlying serratus, and the former may there be cut off. 



The Serratus Posticus (Plate 20). (This and the succeeding muscle 

 were described together by Percivall, under the name superficialis costarum. 

 It corresponds to the serratus posticus inferior of man.) This muscle is 

 provided with an aponeurotic tendon, by which it arises from the 

 summits of the vertebral spines from the 11th dorsal to the 2nd lumbar. 

 The inferior border of the tendon has a muscular fringe with eight or 

 nine distinct slips, which are inserted into the posterior borders and 

 outer surfaces of the same number of ribs at the end of the series. 



Action. — It is a muscle of expiration. 



The Serratus Akticus (Serratus posticus superior of man) (Plate 20). 

 This muscle is partly covered by the preceding, whose three anterior 

 slips should therefore be carefully removed, as has been done in Plate 

 20. It repeats the form of the posticus, having a thin, translucent 

 aponeurotic tendon, which, in front, is confounded with the splenius. 

 By the upper border of this tendon it arises from the summits of the 

 dorsal spines from the 2nd or 3rd to the 13th. The inferior border of 

 the tendon is succeeded by the fleshy portion of the muscle, and this is 

 inserted into the anterior borders and outer surfaces of the ribs from the 

 5th to the 13th inclusive. 



Action. — To assist in inspiration. 



Directions. — The two muscles just described must be removed in 

 order to expose the next layer ; and this is to be done by incising the 

 aponeurotic portion of each horizontally, an inch or two above its point 

 of junction with the muscular portion. The portions above the incision 

 can then without difficulty be stripped upwards from the surface of 

 the longissimus dorsi. The lower portions must next be dissected 

 downwards in order to expose^ the transversalis costarum. In doing 

 this, it will be found that a fibrous septum passes from the aponeurosis 

 of the serratus anticus near its lower border, and, penetrating between 

 the two muscles now exposed, becomes attached to the ribs. This must 

 be cut, and the muscular slips of the serrati must be carefully raised 

 from the transversalis costarum. 



Vessels and Nerves. A set of nerves will be found at the inner 

 edge of the longissimus dorsi, and another perforating its substance. 



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