According to Indian mythology, Dhanvantari is the name of the lord of medicine or precisely the lord of Ayurveda. Lord Dhanvantari is believed to be the incarnation of lord Vishnu and is believed to have originated during samudra manthana (समुद्रमन्थनम्) between devas and asuras. In Ayurveda treatises named Sushruta Samhita, any person who is skilled in the science and art of Surgery has been called as Dhanvantari.
The term Dhanvantari is defined by the commentator Dalhana in his commentary on Sushruta Samhita. According to him,
धनुः शल्यशास्त्रं, तस्य अन्तं पारम् इयर्ति गच्छतीति धन्वन्तरिः (Dalhanas commentary on Sush. Samh. Sutra 1.3) Meaning: Dhanu refers to the science of surgery. The one who who gets mastery over it is therefore known as Dhanvantari i.e. a skilled surgeon.
Therefore specifically in Ayurveda's Sushruta samhita, one of the most ancient encyclopedic and authoritative classical books of Indian medicine, i.e. Ayurveda,, Dhanvantari does not necessarily mean Lord Dhanvanatari but, a skilled surgeon who has mastered the art and science of Surgery. Dhanwantari is also referred to as the teacher of Shalyatantra i.e science of surgery
आयुर्वेदे धन्वन्तरिसंप्रदायः॥ Dhanvantari tradition in Ayurveda
The dissemination of knowledge of Ayurveda at the very beginning in this world happened from 2 different traditions. One of them is known as Atreya Sampradaya through which the medicine dominant knowledge of Ayurveda was transmitted while the other one is known as Dhanvantari sampradaya through which the knowledge of Ayurveda predominantly of Surgical procedures was transmitted. The teachers and masters of these 3 different traditions, transmitted their specialised knowledge Ayurveda to their students and many of them authored the currently refered treatises of Ayurveda like Charaka samhita and Sushruta samhita. Here Charaka Samhita was compiled by Acharya Agnivesha (redacted by Acharya Charaka in later time) by the preaching of expounder Punarvasu Atreya and who belongs to the Atreya traditions whereas Sushuta Samhita was authored by the Acharya Sushruta who received knowledge from Kashiraj Divodasa dhanvantari of Dhanvantari tradition.
In Mahabharata and many puranas like Vishnupuran, Bhagabatpuran, Agnipuran and Haribans - Vayupuran etc. Dhanvatari, the God of Ayurveda is regarded as the incarnation of lord Vishnu.
The theory of the incarnation of Lord Vishnu as Lord Dhanwantari is not found in Vedas, but many Puranas narrate this topic. According to some Puranas Dhanwantari appeared in this world at the time of "Samudra Manthana". This is the mythological fact about the creation of Lord Dhanwantari. But if we go back to the history there are two Dhanwantaries in different times.
- Lord Dhanwantari (भगवान धन्वन्तरिदेवता) who appeared before gods at the time of "Samudra Manthanam." He is also known as Dhanvantari first.
- Dhanvantari (धन्वन्तरिः)
- Kashiraj Divodasa Dhanwantari (काशीराज दिवोदास धन्वन्तरिः) who was born at Kashi and was an eminent surgery specialist and advisor of Sushruta Samhita.
धन्वन्तरिः प्रथमः ॥ Dhanvantati first : Lord Dhanvantari
In Bharatiya Samskrtika parampara, one can find some stories about the emergence of Dhanvantari during Samudra manthana. In Ramayana, while narrating the story of Samudra manthana, emergence of Dhanvantari is appreciated as procurement of one of the gems through churning the milky ocean.
अथ वर्षसहस्रेण आयुर्वेदमयः पुमान्॥ १-४५-३१ उदतिष्ठत् सुधर्मात्मा स दण्ड सकमण्डलुः। पूर्वं धन्वन्तरिर्नाम अप्सराः च सुवर्चसः॥ १-४५-३२ 
Meaning: After a thousand years of churning had rolled on, there arose a male being, an embodiment of Ayurveda, highly righteous, named Dhanvantari with a stick in his hands and a Kamandalu. ..
Reference of emergence of Dhanvanatari from Samudra manthana can be found in the Mahabharata, Vishnupurana, Vayupurana and Bhagawatapurana as well.
Emergence of Dhanvantari during Samudra manthana
Once there arose a great problem to save the world from danger. The God was puzzled seeing this terrible situation. Trinnath Sabha was arranged. Brahma, Vishnu and Maheswar considered the matter and told that great injustice has been done to Gods, who have been deprived of getting nectar at the time of churning of milk sea. If it will be churned again ordure will come out as a result of which the whole world will be annihilated. On the other hand immortal Gods are in the distress being wounded and caught by many diseases. For the settlement of peace, treatment and to save he world from danger, Dhanwantari was sent for. Accompanied by his colleagues he appeared in the "Trinath Sabha". He proposed that "we can get nectar from the trees given by Laxmi, the daughter of sea instead of churning the sea." We can get plenty of it from the trees of mountain Gandhamardan. If order will be given, my descendant will bring these threes. Drinking their nectar many people will be cured from their diseases. Although they will not be immortal, they will be long-lived. This proposal of Dhanwantari was unanimously accepted. At that time Sukracharya knew Sanjivani Mantra of animal as Dhanwantari knew Sanjivani Mantra of plant. But animals depend upon trees for their survival. So trees accepted Dhanwantari as their Guru. The moment, they get order from Dhanwantari, they immediately bear fruits, flowers and confer juice. Then Dhanwantari collected juice from soma trees, palm trees and also from many other trees of Gandhamardan. His descendants conferred it to Gods and demons. Drinking the juice of those trees they were cured from their diseases and became strong and stout. But stupid demons concealed some of it. They drank making it bear at a secret place. No doubt they were cured from their diseases became healthy, but there was no development of their mental power. They became tyrant and quarrelsome so long as they would not discard drinking bear.
In Vedic period Aswinis were expert medical practitioners and in Puranic period Dhanwantari became the same as Aswinis.
Guru of lord Dhanvantari
In Matsyapurana, there is a reference which says that, Dhanvantari obtained Medicine knowledge from Bhaskara. Dhanvantari emerged from Samudra manthana is said to be possessed by Bhaskara. Therefore as per opinion of few scholars, Bhaskara is said to be the guru (Preacher ) of Dhanvantari.
गजेन्द्रञ्च सहस्राक्षो हयरत्नं च भास्करः । धन्वन्तरिञ्च जग्राह लोकारोग्यप्रवर्तकम्॥ (Mats. 251/4)
Bhaskara, the guru of Dhanvantari is known to have written a treatise named Bhaskarasamhita, on th ebasis of which his disciples including Dhanvantari are said to have written their own separate treatises. In Brahmavaivarta purana, Dhanvantari is said to have written a treatise named as 'Chikitsatatvavijnana' (चिकित्सातत्वविज्ञानम्). 
धन्वन्तरिः द्वितीयः॥ Dhanvantari second
By the term Dhanvantari second, we mean that Dhanvantari, who was born in the fourth or fifth generation in Sunhotra's lineage, in the Chandravanshi rajakula of Kashi (Varanasi). We can find the family tree of this Dhanvantari in Harivansha and other puranas.  . There is a slight difference in the geneology found in these 4 references.
- Some consider Dhanva to be the son of Dirghatapa and Dhanvantari to be the son of Dhanva, while others consider Dhanvantari to be the son of Dirghatapa directly.
- In Bhagavata purana (Skanda 9A/17) and the Garuda purana, the son of Dirghatapa Dhanvantari has been considered to be the originator of Ayurveda. The adjective scholar has been applied to this Dhanvantari. He has been called Sarvarogapranashan (Destroyer of all diseases) and Ayurveda promoter (Bhagwatpurana). From all this it is understood that this Dhanvantari must had been the expert, well-versed and knowledgeable in all disciplines of Ayurveda. It is thus possible that he was named Dhanvantari because of his expertise in surgery.
- In some puranas one can find the story about the origin of Dhanvantari. In Harivans- Vayupurana and Brahma Baivarta Purana, it has been described that Lord Vishnu sent Dhanwantari, as the son of king Dhanwa of Kashi. According to the Harivans- Vayupurana, the king of Kashi named 'Dhanva' was a great devotee of lord Vishnu. He had everything in life except a progeny. He worshipped lord Vishnu for this purpose and Lord Vishnu was impressed with his devotion and prayers. Thus Lord Vishnu took birth as son of the king Dhanva of Varanasi in Bharata. Being the son of Dhanva and incarnation of lord Vishnu, son of Dhanva was surnamed as Dhanvantari. 
कालः॥ Time period of this Dhanvantari
The Time period of this Dhanvantari is believed to be approximately 5044 BC.
धन्वन्तरिः तृतीयः॥ Dhanvantari third : Kashiraj Divodas Dhanvantari
Name of this Dhanvantari is taken as father of Shlyapradhana i.e. Surgery dominated tradition of Ayurveda. The prestige of Dhanvantari tradition of Ayurveda is the result of his efficiency and expert work. He is famous in the Ayurveda fraternity as a surgery expert. Divodas was the founder of Varanasi city. The tradition of Ayurveda has remained intact in Kashiraj's clan and it has been propagated in every era. Divodas enriched that tradition. He started teaching Ayurveda in the institutionalized form. Disciples from distant countries used to come to his place for learning ayurveda. Among the disciples of Divodasa, Sushruta, Aupadhenava, Vaitarana, Aurabhra, Paushkalavata, Karavirya and Gopurarakshita have been named. 
At various places in Sushruta samhita, Dhanvantari is praised using number of adjectives. From all these it is understood that Dhanvantari was a great ascetic, scholar of scriptures, a pious person and a man of liberal attitude. Some specific epithets given to Divodasa Dhanvantari in Sushruta Samhita state that, he was the scholar of Ashtanga Ayurveda and thus was the remover of the doubts related to the meaning of scriptures. He was formerly a devavidya (Physician of Gods) an dhe cured the deities of old age & death and made them immortal and disease free. HE incarnated in human form for transmitting the knowledge about surgical Ayurveda in mortal world. As he is found describing anad introducing himself at one place in Sushruta samhita as below,
अहं हि धन्वन्तरिरादिदेवो जरारुजामृत्युहरोऽमराणाम् | शल्याङ्गमङ्गैरपरैरुपेतं प्राप्तोऽस्मि गां भूय इहोपदेष्टुम् ||२१|| (Sush Samh 1.21) Meaning: I am Dhanvantari, who destroyed ageing, death and pain of Devatas (gods). I have come here to teach the (art and science behind this medicinal system) knowldge of same equipped with all limbs like Surgery etc.
कालः॥ Time period of this Dhanvantari
The time period of this Dhanvantari is calculated to be approximately 1000-1500 BC.
Guru of Dhanvanatri
In Sushruta Samhita, there is a verse that describes from where Dhanvantari acquires knowledge of Ayurveda.
ब्रह्मा प्रोवाच ततः प्रजापतिरधिजगे, तस्मादश्विनौ, अश्विभ्यामिन्द्रः, इन्द्रादहं, मया त्विह प्रदेयमर्थिभ्यः प्रजाहितहेतोः ||२०|| (Sush Samh 1.20)
From this it is understood that Dhanvantari acquired knowledge of Ayurveda from Indra. For the wellbeing of mankind on this earth he asked for this knowledge to Indra and brought it into this world.
As per Harivansha (Parva 1 Adhyaya 29), Dhanvantari obtained knowledge from Bharadwaja Rshi who has been known to acquire it from Indra first.
Treatises contribution by Dhanvantari
According to Brahmavaivarta Khanda Adhyaya 16, Dhanvantari has authored various texts, few to mention among those are Chiktsadarshan and Chikitsakaumudi.
प्राचीनता ॥ Antiquity
Dhanvantari's mentions in Mahabharata, Harivanshpuran and Vayupuran etc.; In the Pali book named Milindapanho and the Jataka book named Ayoghar; mentions of the Harivamsa of Divodas as the son of Bhimsen or Bhimaratha, in the Mahabharata and the Kathakasamhita; mention as the father of Pratardan of Divodas in Kaushitaki Brahmin, Kaushitaki Upanishad, Katyayaniyya-Riksarvanukram and Mahabhashya and due to mention of events like establishment of Varanasi by Divodas in Mahavagga etc. Dhanvantari is believed to be an Upanishad-time Acharya .
Dorothea Chaplin has considered the time of Dhanvantari to be 1200 years before the time of Hippocrates. Akshay Kumar Majumdar has considered the time of Dhanvantari as 1600 BC, Bhimrath's son Divodas as 1500 BC.
In the wars of Ramayana and Mahabharata, their quotes were required for the surgery of the injured persons and for pricking arrows etc. and for that surgery. Shlyakarma was done. Surgery has been going on since time immemorial. The ancientity of the Dhanvantar-sampradaya is unquestionably valid due to the many quotes of Bhagnasandhan in Rigveda and Atharvaveda. Since the description of Dhanvantari Divodas etc. is available in the Puranas, their antiquity is self-evident.
धन्वन्तरिजयन्तिः ॥ Dhanvantari Jayanti
The Birth Anniversary of Dhanvantari devata is celebrated in the month of Kartika on Tithi Trayodashi. It is popular as Dhanvantari Jayanti or Dhanteras or Dhanatrayodashi all over the country.
It is stated that, Sushruta the author of Sushruta Samhita received the knowledge of Ayurveda and specifically Shalya (Surgery in Ayurveda) from the abovementioned Kashiraj Divodasa Dhanwantari . Thus, Dhanvantari is referred to as the teacher of Shalyatantra or science of major surgery.
भगवानधन्वन्तरेः स्वरूपम॥ Appearance of Lord Dhanvantari
Many Hindu mythologies reveal his appearances that Lord Dhanwantari has four hands. In one hand, there is a nectar pot, in another there is a Ayurvedic book, a medicinal herb in the third hand and in the fourth hand there is a conch shell. Equipped with these things, he appeared in this world to save mankind in their adversity. In some pictorial presentations of Bhagwan Dhanvanatari, one of his hands is shown holding a leech. Leech is a slimy blood sucking ectoparasite that lives in water and is used as a medium for therapeutic blood letting procedure known as Raktamokhana in Ayurveda.
धन्वन्तरेः कार्यम् ॥ Works of Dhanvantari
Dhanwantari is known master of healing all diseases. He was taught by Rishi Bharadwaja (Or Indra directly) and first time divided Ayurveda into 8 parts and transferred it to his students.
Dhanwantari was master of all branches of Ayurveda. He was an eminent toxicologist. On the other hand he was also well-known physician of Horses (Ashwayurveda) and elephants (Hastyayurveda). Thus Dhanwantari developed 8 branches of Ayurveda at his period, especially the surgery by the time Dhanwantari became the surgery specialist and Dhanwantari group widely known as surgery specialist i.e. "Salya Sampradaya". However it is undoubtedly accepted that Dhanwantari was the incarnation of Lord Vishnu. 
- Ramayana, Balakanda, Sarga 45.
- Dr. Dinabandhu Moharana. 78 Orissa Review July - 2008.
- Matsyapurana 251/4
- Brahamvaivartapurana (Brahamkhanda Adhyaya 16)
- Harivansha purana (1|32|18-22,28)
- Harivansha purana (1|29|5-10)
- Brahmanada purana (2|66|3)
- Vayupurana (92|18)
- Harivansha Purana(1.29)
- Ayurveda ka itihas ewam parichay (Prushtha 39-42)
- Sushruta Samhita (Sutrasthanam Adhyaya 1 Sutra 3)
- Sushruta Samhita (Sutrasthanam Adhyaya 1 Sutra 20-21)