IGO 



Having thus seen the method in which the muscles in a typical 

 extremity are arranged, it becomes, in the next place, a point of interest 

 to determine whether there is any such definite order in the arrange- 

 ment of the trunk muscles as we find to be present in the limb. 



Before doing this we have to determine what muscles there are in 

 the body uniting the typical limb to the true axis, and these are 

 named below : the most interesting of these are two — one arising from 

 the haemal arches, and inserted into the vertebral margin of the basal 

 bone, developed in the forelimb as the serratus magnus ; in the hinder 

 extremity, as the psoas ; both of these agree in their typical origin and 

 insertion, for the so-called transverse processes of the lumbar vertebrae, 

 to which the latter is attached, are in reality bases of rudimentary 

 haemal arches. Secondly, we have a muscle lying along the verte- 

 bral border of the last described which arises from the transverse 

 processes, and is attached to the upper and inner angle of the basal 

 bone, near the iliacus, or subscapularis. This muscle, in the upper 

 limb of man, is the levator scapulae, so often continuous with the 

 serratus magnus. In the hinder limb, this muscle is also repre- 

 sented by the psoas — a muscle very often in the animal kingdom di- 

 vided, and devoted partly to another and different purpose. The 

 psoas parvus (the largest portion in " Echidna," loc. cit. p. 389), is 

 the true index of this muscle in its typical position ; but its differen- 

 tiated portion, called the psoas magnus, in man is, by being extended 

 into a common tendon with the iliacus, rendered a powerful accessory for 

 the flexion of the leg. This theoretic explanation gives us the proper 

 clue to the nature of the psoas parvus — a muscle whose affinities are 

 otherwise difficult to be understood, and whose action must, in its usual 

 human condition, be very limited. 



Eemoving these muscles from the trunk, we find the true body 

 muscles remaining, and to their nature there is the clue to be found in 

 the arrangement of the bony skeleton; for as the osseous axis of the 

 body is made up of a series of vertebrae, and their appendages, so it is 

 but natural to expect the soft parts to be built upon a basis of the same 

 kind ; and accordingly, when we examine the muscles of the trunk, 

 they can be reduced to a system of vertebral appendages ; of inter- 

 vertebral and intercostal muscles. Of these, the most regular groups 

 are to be found in the thoracic region, and there we can resolve them 

 into several groups. I select the thorax as the most typical region, 

 because there we have the greatest amount of regularity in the osseous 

 framework, and the greatest degree of uniformity of function among the 

 different component muscles. 



Having culled from the thoracic group all those muscles which are 

 not truly parts of the trunk system, but which form parts of the upper 

 limb, we find that there are five distinct types remaining, two series 

 of intercostals, an interna] -sternal transversus thoracis, and an internal 

 vertebral transversus thoracis, and a straight vertical muscle, the 

 rectus thoracicus; these we find to have each a definite direction, 

 and series of attachment ; and when we compare the other regions 



