Cardiocentric hypothesis

According to the cardiocentric hypothesis, the heart has a profound influence on human emotions, cognition, and awareness. This notion may be traced back to ancient civilizations such as Egypt and Greece, where the heart was regarded not only as a physical organ but also as a repository of emotions and wisdom. Aristotle, a well-known Greek philosopher in this field, contributed to the notion by thinking the heart to be the centre of both emotions and intellect. He believed that the heart was the center of the psycho-physiological system and that it was responsible for controlling sensation, thought, and body movement. He also observed that the heart was the origin of the veins in the body and that the existence of pneuma in the heart was to function as a messenger, traveling through blood vessels to produce sensation. This point of view remained throughout history, spanning the Middle Ages and Renaissance, influencing medical and intellectual debate.

An opposing theory called "cephalocentrism", which proposed that the brain played the dominant role in controlling the body, was first introduced by Pythagoras in 550 BC, who argued that the soul resides in the brain and is immortal. His statements were supported by Plato, Hippocrates, and Galen of Pergamon. Plato believed that the body is a "prison" of the mind and soul and that in death the mind and soul become separated from the body, meaning that neither one of them could die.

History
In ancient Egypt, people believed that the heart is the seat of the soul and the origin of the channels to all other parts of the body, including arteries, veins, nerves, and tendons. The heart was also depicted as determining the fate of ancient Egyptians after they died. It was believed that Anubis, the god of mummification, would weigh the deceased person's heart against a feather. If the heart was too heavy, it would be considered guilty and consumed by the Ammit, a mythological creature. If it was lighter than the feather, the spirit of the deceased would be allowed to go to heaven. Therefore, the heart was kept in the mummy while other organs were generally removed.

However, the ancient Greeks, Aristotle promoted the cardiocentric hypothesis based on his experience with animal dissection. He found that certain primitive animals could move and feel without the brain, and so deduced that the brain was not responsible for movement or feeling. Apart from that, he pointed out that the brain was at the top of the body, far from the centre of the body, and felt cold. He also performed anatomical examinations after strangling the specimen, which would cause vasoconstriction of the arterioles in the lungs. This likely had the effect of forcing blood to engorge the veins and make them more visible in the following dissection. Aristotle observed that the heart was the origin of the veins in the body, and concluded that the heart was the centre of the psycho-physiological system. He also stated that the existence of pneuma in the heart was to function as a messenger, traveling through blood vessels to produce sensation. Movement of body parts was thought to be controlled by the heart as well. From Aristotle's perspective, the heart was composed of sinews which allowed the body to move.

In the fourth century BC, Diocles of Carystus reasserted that the heart was the physiological centre of sensation and thought. He also recognised that the heart had two cardiac ears. Although Diocles also proposed that the left brain was responsible for intelligence and the right one was for sensation, he believed that the heart was dominant over the brain for listening and understanding. Praxagoras of Cos was a follower of Aristotle's cardiocentric theory and was the first one to distinguish arteries and veins. He conjectured that arteries carry pneuma while transporting blood. He also proved that a pulse can be detected from the arteries and explained that the arteries' ends narrowed into nerves.

Lucretius stated around 55 BCE, "The dominant force in the whole body is that guiding principle which we term mind or intellect. This is firmly lodged in the midregion of the breast. Here is the place where fear and alarm pulsate. Here is felt the caressing touch of joy. Here, then, is the seat of the intellect and the mind."

The Islamic philosopher and physician Avicenna followed Galen of Pergamon, believing that one's spirit was confined in three chambers of the brain and accepted that nerves originate from the brain and spinal cord, which control body movement and sensation. However, he maintained the earlier cardiocentric hypothesis. He stated that activation for voluntary movement began in the heart and was then transported to the brain. Similarly, messages were delivered from a peripheral environment to the brain and then via the vagus nerve to the heart.

In the Middle Ages, the German Catholic friar Albertus Magnus made contributions to physiology and biology. His treatise was based on Galen's cephalocentric theory and was profoundly affected by Avicenna's preeminent Canon, which itself had been influenced by Aristotle. He combined these ideas in a new way which suggested that nerves branched off from the brain but that the origin was the heart. He concluded that philosophically, all matters originated from the heart, and in the corporeal explanation, all nerves started from the brain.

William Harvey, an early modern English physiologist, also agreed with Aristotle's cardiocentric view. He was the first to describe the basic operation of the circulatory system, by which blood was pumped by the heart to the rest of the body, in detail. He explained that the heart was the centre of the body and the source of life in his treatise De Motu Cordis et Sanguinis in Animalibus.

In major Abrahamic religious texts
Both the Bible, in the Old Testament and New Testament, and the Quran make statements that the heart is the location of thoughts, beliefs and intentions, consistent with the cardiocentric hypothesis. Neither text explicitly or implicitly mentions the brain or even the head as the location of thoughts or conscious processes.

Quran
The Quran mentions the heart in connection with thoughts, beliefs, intentions in numerous verses including:

26:194 Truly it has been sent down by the Lord of all the worlds, the Trustworthy Spirit has alighted with it upon your heart [O Prophet], that you may be one of the Warner.

39:23 ''God has sent down the best Message: a Scripture that is consimilar and oft-repeated: that causes the skins of those in awe of their Lord to creep. Then their skins and their hearts soften at the mention of God: such is God's guidance. He bestows it upon whoever He will; but no one can guide those whom God leaves to stray.''

2:74 ''Even then your hearts became hardened like a rock or even harder, for some rocks gush rivers; others split, spilling water; while others are humbled in awe of Allah. And Allah is never unaware of what you do.''

7:179 ''Indeed, We have destined many jinn and humans for Hell. They have hearts they do not understand with, eyes they do not see with, and ears they do not hear with. They are like cattle. In fact, they are even less guided! Such people are entirely heedless.''

5:41 ''O Messenger! Do not grieve for those who race to disbelieve—those who say, “We believe” with their tongues, but their hearts are in disbelief. Nor those among the Jews who eagerly listen to lies, attentive to those who are too arrogant to come to you. They distort the Scripture, taking rulings out of context, then say, “If this is the ruling you get ˹from Muḥammad˺, accept it. If not, beware!” Whoever Allah allows to be deluded, you can never be of any help to them against Allah. It is not Allah’s Will to purify their hearts. For them there is disgrace in this world, and they will suffer a tremendous punishment in the Hereafter.''

50:37 In that is a reminder for whoever possesses a heart, or cares to listen and witness.

Old Testament
The Old Testament mentions the heart on many occasions in the context of thoughts, actions, beliefs and knowledge including:

...He is wise in heart, and mighty in... Job 9:4 ...ye know in all your hearts and in all... Josh 23:15 ...''and apply thine heart unto my knowledge... Prov 22:17'' ...I applied my heart to know wisdom... Eccl 8:16 ...consider in thine heart, that the... Deut 4:39 ...understand with their heart, and convert... Is 6:10 ...thine heart shall meditate terror... Is 33:18

New Testament
The New Testament mentions the heart on many occasions in the context of thoughts, actions, beliefs and knowledge including:

...and pondered them in her heart... Luke 2:19 ...perceiving the thoughts of their hearts... Luke 9:47 ...''But what comes out of the mouth proceeds from the heart, and this defiles a man. For out of the heart comes evil thoughts, murder, adultery, fornication, theft, false witness''... Mt 15:17–20 ...and pondered them in her heart... Luke 2:19 ...perceiving the thoughts of their hearts... Luke 9:47

Cephalocentric perspective
Hippocrates of Kos was the first to suggest that the brain was the seat of the soul and intelligence. From his treatise De morbo sacro, he pointed out that the brain controls the rest of the body and is responsible for sensation and understanding. Apart from that, he believed that all feelings originated from the brain.

Galen of Pergamon was a biologist and physician. His approach to the investigation of the brain was due to his rigorous anatomical methodology. He pointed out that only correct dissection will support the incontrovertible statement. He reached the conclusion that the brain was responsible for sensation and thought, and that nerves originated at the spinal cord and brain.

Brain in heart
The "little brain in the heart" is an intricate system of nerve cells that control and regulate the heart's activity. It is also called the intrinsic cardiac nervous system (ICNS). It consists of about 40,000 neurons that form clusters or ganglia around the heart, especially near the top where the blood vessels enter and exit. These neurons communicate with each other and with the brain through chemical and electrical signals.

The intrinsic cardiac nervous system has several functions, such as:


 * Adjusting the heart rate and rhythm according to the body's needs and emotions.
 * Sensing and responding to changes in blood pressure, oxygen levels, hormones, and inflammation.
 * Protecting the heart from damage during a heart attack or other stress.
 * Learning and remembering from past experiences and influencing the brain’s memory and emotions.