שמואל ב ב-כג

וַיְמָאֵן לָסוּר וַיַּכֵּהוּ אַבְנֵר בְּאַחֲרֵי הַחֲנִית אֶל הַחֹמֶשׁ

וַתֵּצֵא הַֽחֲנִית מֵאַחֲרָיו וַיִּפָּל שָׁם וַיָּמָת  תַּחְתָּיו  

וַיְהִי כָּל הַבָּא אֶֽל הַמָּקֹום אֲשֶׁר נָפַל שָׁם עֲשָׂהאֵל וַיָּמֹת וַֽיַּעֲמֹֽדוּ׃

2 Samuel 2:23 Howbeit he refused to turn aside: wherefore Abner with the hinder end of the spear smote him under the fifth rib, that the spear came out behind him; and he fell down there, and died in the same place: and it came to pass, that as many as came to the place where Asahel fell down and died stood still.

The encyclopedia in describing the heart uses as part of that description this: “the apex [of the heart] touches the chest wall between the fifth and sixth ribs.” Apex is the highest point. So the fifth rib would be the most vulnerable point of entry into the heart.

Folio 49a Then Joab was brought before the Court,²  and he [Solomon] judged and questioned him, 'Why did thou kill Abner?'³  He answered, 'I was Asahel's⁴  avenger of blood.'⁵  'But Asahel was a pursuer!'⁶  'Even so,' answered he; 'but he [Abner] should have saved himself at the cost of one of his [Asahel's] limbs.'⁷  'Yet perhaps he could not do so, remonstrated [Solomon]. 'If he could aim exactly at the fifth rib,' he retorted, ('even as it is written, Abner with the hinder end of the spear smote him at the waist;⁸  concerning which R. Johanan said: It was at the fifth rib, where the gall-bladder and liver are suspended.) — could he not have aimed at one of his limbs?' Thereupon [Solomon] said: 'Let us drop [the incident of] Abner; why didst thou kill Amasa?'⁹  He answered: 'Amasa disobeyed the royal order,¹⁰  for it is written, Then said the King to Amasa, Call me the men of Judah together within three days etc. So Amasa went to call the men of Judah together; but he tarried etc.' 'But,' said he [Solomon], 'Amasa interpreted [the particles] 'Ak and Rak.'¹¹  [Thus:] he found them¹²  just as they had begun [the study of] a tractate; whereupon he said: It is written, Whosoever he be that shall rebel against thy [the King's] commandments and shall not hearken unto thy words in all that thou commandest him, he shall be put to death.¹³  Now, one might have thought that this holds good even [when the transgression is committed] for the sake of the study of the law: it is therefore written, only [Rak] be strong and of good courage.¹⁴  But thou thyself¹⁵  didst disobey the royal order, for it is written, And the tidings¹⁶  come to Joab, for Joab had turned after Adonijah, though he had turned not after Absalom.¹⁷  What is the purpose of 'though he had turned not.'¹⁸  — Rab Judah said: He wished to turn [after him], but did not. And why did he not? — R. Eleazar said: David still possessed his vitality.¹⁹  R. Jose the son of R. Hanina said: David's star²⁰  was still in the ascendant, for Rab Judah said in Rab's name:²¹  Four hundred children had David, all the issue of yefoth to'ar; they had long locks, and used to march at the head of the troops; it was they who were the men of power in David's household.

Gray's Anatomy of the Human Body

Heart.—The outline of the heart in relation to the front of the thorax (Figs 1216, 1218) can be represented by a quadrangular figure. The apex of the heart is first determined, either by its pulsation or as a point in the fifth interspace, 9 cm. to the left of the midsternal line

see Pectoralis major muscle

see Sternum

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