1. The M. transversus thoracis in both man and monkey arises from the rib in the area extending from the lateral tip of the costal cartilage to the bony rib, that is, from the rib or costal cartilage near costochondral joint, and inserts into the lateral edge of the sternum or xiphoid process.
In man, however, there were instances in which this muscle arose from the intercostal space, that in which it inserted into the sternocostal joint, etc., so that depending upon the relation between the origin and insertion a classification into four types was possible. A further subdivision was possible according to whether the origin or insertion is by a single muscle or separated into two parts.
2. The rib of origin in monkey ranges from the first to seventh rib, but the origin from the first rib was frequently absent. In man it was more limited both above and below, usually being from the second to sixth rib.
3. This muscle in monkey was well developed so that at the insertion the muscular digitation united each other to form a triangular muscular sheet with the tip being the site of origin at the rib and the base being that of insertion into the sternum. In man, however, fusion at the i n sertion and thus the formation of a muscular sheet was found only for those muscles arising from the lower ribs.
4. This muscle in man and monkey mostly crossed over one rib to occupy two intercostal spaces.
5. The location of this muscle corresponds to the area in which the M. intercostalis internus is absent so that it appears to compensate the region where the M. intercostalis internus is absent.