18. Inspection with the flesh of the finger and also the fingernail. According to the Gemara (חולין דף י"ז ע"ב ), one must check the knife with the flesh of the finger and also with the fingernail [see picture, courtesy of Harav Amitai Ben David, mechaber of Sichas Chullin]. We use the flesh of the finger because the esophagus is made from fleshy tissue, so if the finger feels a nick, the esophagus will feel it too. Similarly, the knife must be checked with the fingernail because the trachea is made of cartilage, which is a stiffer substance, comparable to the material of the fingernail. If a nick is not felt by the nail, it will not cause the shechita of the esophagus to be posul (תוס' חולין דף י"ז ע"ב ד"ה אבישרא, ט"ז סי' י"ח סק"ו ).
19. Today. Today, we only check with the nail, not the flesh of the finger (ס' נחפה בכסף ח"ב קו' פרי מפרי, שו"ת מהר"ם שיק יו"ד סי' ו'). This is because checking with the nail only helps with a knife of average sharpness. When a knife is extremely sharp, as they are today, if one would properly touch the blade to do a meaningful inspection, it would be impossible not to cut the finger (דרכ"ת סק"ג ). This is the minhag of all shochtim today, even the ones with tremendous yiras Shomayim and the strictest standards (דרכ"ת שם ).
20. Some say one can fulfill the mitzva of Chazal to check on the flesh even today by checking on one’s flesh whether the knife is sharp; this is the minhag of shochtim and bodkim (דעת תורה סקי"ט, מנח"י בלק"י סק"ג ). However, this does not fulfill 6 of the inspections (see 21 below).
