The _Sacrolumbus_ hath its Source in the hinder part of the _Os Sacrum_, as also in the _Vertebra's_ of the Loins, and ascending from thence, insinuates it self into the hinder part of the Ribs, to every one of which it imparts two _Tendons_, one whereof sticks on the outside, and the other on the inside. This Muscle is fleshy within, and fibrous without. The Eleven _Internal Intercostals_, contrary to the External, derive their Original from the {33} top of every lower Rib, and ascend obliquely from the back-part to the fore-part, till they are join'd to the lower Lip of every upper Rib: Thus these Internal Muscles, with the External, form, by the opposition of their Fibres, a Figure resembling a _Burgundian_ Cross. The Diaphragm or Midriff is esteem'd as the fifty seventh Muscle of the Breast, and serves as well for its dilatation as contraction. It separates the _Thorax_ or Chest from the lower Belly, and is tied circularly to all the Extremities of the Bastard Ribs, immediately under the _Xiphoides_, or Sword-like Cartilage. Modern Anatomists have discover'd that the Diaphragm is compos'd of two Muscles, _viz._ one Upper, and the other Lower; so that the Upper cleaves to the Extremities of the Spurious Ribs, and is terminated in a flat _Tendon_ in the middle, which hath been always taken for its Nervous part. The Lower begins with two Productions, the longest whereof being on the right side, ariseth from the three upper _Vertebra's_ of the Loins, and the other on the Left from the two _Vertebra's_ of the Back, till it is lost in the _Aponeurosis_ of the Upper Muscle. _How many Muscles are there in the Back and the Loins, and which be they?_ There are three in each side, _viz._ one for Flection, and the other for Extension. The _Triangularis_ is the _Flexor_, taking its rise in the hinder part of the Rib of the _Os Ilion_, and the inner part of the _Os Sacrum_, in passing from whence it is joined to the last of the {34} Bastard Ribs, and to the transverse Productions of the _Vertebra's_ of the Loins. The _Extensors_ are the _Sacer_, and the _Semi-spinatus_, which make the Waste streight, and are so interwoven along the Back-Bone, that one would imagine that there were as many Pairs of Muscles as _Vertebra's_, affording _Tendons_ to all. The _Sacer_ springs from behind the _Os Sacrum_, as also from the hinder and upper Extremity of the _Os Ilium_, and is inserted in the Spines of the _Vertebra's_ of the Loins and Back. The _Semi-spinatus_ hath its Source in the Spines of the _Os Sacrum_, and is join'd to all the transverse Productions of the _Vertebra's_ from the Back to the Neck, being exactly situated between the _Sacer_ and the _Sacrolumbus_. * * * * * CHAP. IX. _Of the Myology, or Anatomy of the Muscles of the lower Belly._ _How many Muscles are there in the lower Belly, and which be they?_