When pope Felice IV dedicated to the Saints Cosmas and Damian one of the earliest churches to rise up in the Roman Forum, two oriental saints were finally deemed to deserve the same honor as the Roman martyrs. In that space fraught with the history of ancient Rome, the apsidal mosaics lasted as a true inspiration to the following medieval decorations.
A church bloomed in the Roman Forum
The Ostrogothic king Theodoric the Great and his daughter Amalasunta  granted pope Felice IV (526-530) the so-called Templum Divi Romuli on the Via Sacra and the ancient hall of Vespasian near the Templum Pacis, which served as the library of the Forum Pacis,  then as prefect archives and land registry of the city.  The hall was restored at the time of Septimius Severus, who placed there the Forma Urbis Romae, a marble plan of the city of Rome. In the 4th century, the hall was expanded with the addition of a rotunda, dated to the time of Maxentius and variously identified with the Templum Divi Romuli or the Templum Urbis Romae. Following the building of an apse, the hall spaces were mostly used for the basilica and the sacristy, whereas the rotunda served as the vestibule of the basilica. 
You find it here
In 526 pope Felice IV consecrated the church,  dedicating that ancient building complex to the Saints Cosmas and Damian who were considered twin brothers, both physicians and martyred under Diocletian. They were called “Holy Unmercenaries” (from the ancient Greek Ανάργυροι, which can be translated with “anargiri” or “without money”, deriving from their refusal to accept any payment for their services). Cosmas and Damian were locally venerated in Cyrrhus area, where they were buried, then their worship circulated until the two oriental saints were deemed to deserve the same honor as the Roman martyrs, thus manifesting also the sought-after universality of the Roman church. Furthermore, pope Felice intended perhaps to pay a diplomatic homage to Theoderic, besides a possible act of faith caused by the fear of the plague coming from the East. The dedication of the church in this area could be finally explained with the conviction that the worship of Apollo, the pagan god of medicine, was practiced in ancient times in the nearby, where physicians were supposed to hold their meetings and even the well-known Galen was believed to dwell there. 
Therefore, a church dedicated to the healer oriental saints bloomed in the Roman Forum. Belonging to the typology of the urban sanctuary, it was the destination of the Christian pilgrims who hoped to recover their bodies’ health.  Pope Felice IV decided to kept the classical marble decoration on the inner walls and in those spaces fraught with history the pope commissioned the mosaic decorations, on the apse and perhaps on the arch.  A precious help to retrace the original aspect of the basilica is still offered by Onofrio Panvinio and Pirro Ligorio’s drawings, sketched on the site when the basilica wasn’t yet altered due to the 17th century works.
The blue background mosaic
In the apse vault mosaic, an innovative iconography shows the presence of the titular Saints Cosmas and Damian while they're entering the eternal spheres, together with the commissioning pope. The martyrs are guided by the Saints Peter and Paul as they bring two wreath crowns, symbols of their victory over the death, to offer them to Christ,  whereas pope Felice IV offers to the Saviour the model of the church, represented with the atrium yet missing the tower bell.  In a vibrant theophany, Christ comes down a “stairway made of clouds”  from the Empyrean Heaven against a blue sky, flaming at an evocative dawn. An act of protection tighten the two symmetrical groups of apostles and martyrs in a landscape setting which is uplifted in a transcendental representation.  Only Christ has the head wreathed with the nimbus and this proves that the usage of wreathing the saints’ heads with this symbol of glory wasn’t yet frequent in the early 6th century.
The author of the apse mosaic worked for a vigorous blend of motifs, giving them a new significance. The seven-figures composition hails from the usual worship halls, but the artist chose the moment of the martyrs’ entry to the Empyrean Heaven, when the two principal apostles, Saints Peter and Paul, introduce Saints Cosmas and Damian to Christ with the same patronage gesture as that one represented in the late cemetery painting. The figure of the commissioning pope, depicted while he’s bringing the model of the church, is maybe inspired by a motif already appeared on the arch of the basilica di San Pietro in Vaticano.  This figure has been considered as one of the first images of a commissioning pope ever appeared in a public decoration in Rome.  The iconographic theme of the traditio legis (the delivery of the law) inspired the figure of Christ floating above the ground level as well as the paradisiacal setting, representing the Jordan River, the palm trees and the phoenix.
The legendary phoenix, with the head surrounded by the brilliance of a star, is an allegory of immortality. Christians inherited it from the pagan mythology: Herodotus and Plinius bequeathed the legend about its immortality. The long lived bird was depicted on the imperial coins dating to Hadrian’s period but already on the monuments of ancient Egypt the phoenix evoked a mythic era, called “Sothic cycle” or “Canicular period”, lasting 1461 years.
The artist finally succeeded in giving the image still a concreteness, mindful of the close classical past, as evident in the individuation of the time, the almost naturalistic place and the lively characterization of the monumental figures. The blue background powerfully contrasts with the golden drapery of Christ’s clothing as well as the apostles’ candid ones. The chiaroscuro shadows have disappeared yet the colors have the constructive function of modelling the image shapes and, in the chromatic combination, each hue is rationed in order to shine at its best intensity. 
Below the figurative representation, a procession of lamps is depicted in a narrow register, closed by another blue-background band along the rime of the apse conch. The hexameter golden inscription in capital letters highlights the two titular saints and the commissioning pope, praising the visual rendering of the bright decoration:  “Aula di claris radiat speciose metallis / in qua plus fidei lux pretiosa micat / martyribus medicis populo spes certa salutis / venit et ex sacro crevit honore locus / optulit hoc dno felix antistite dignum / munus ut aetheria vivat in arce poli”.  At a lower stage, a later series of Saints and Blessed belonging to the Third Order of St. Francis runs above the 17th century wooden choir.
The mosaic has undergone several retouching and restoration interventions since at least the times of pope Gregorio XIII (1572-1585):  for example, the figure of pope Felice IV is completely remade and the head of Christ is very much restored.  The now lost presence of God the Father’s hand giving Christ a victory crown is proved by a 1699 drawing by Giovanni Giusto Ciampini, after Narciso Spina. 
The arch mosaic
The elements of the first vision of the Apocalypse are represented on the apsidal arch in a sacred tone, with the substitution of the Anonymous with the Lamb.  On a golden background, some of the Twenty-Four Elders raise the crowns on their covered hands toward the Lamb on the apex, who seats on a gemmed throne adorned with a cross and carrying the sealed scroll. Next to the Lamb, seven golden candlesticks are followed by two angels, who proceed on the sea of glass, accompanied by one of the Four Living Creatures on each side: the Angel and the Eagle, who respectively carry a precious book and can be therefore identified with the evangelists Matthew and John. Both sides of the arch decoration have been unfortunately cut off due to the building of the side chapels in the early 17th century, causing the vanishing of the other two Living Creatures and the rest of the Twenty-Four Elders. 
The composition of the figures draws inspiration from the earlier monuments and miniature imagery and shows the mature assimilation and genuine reaction in Rome’s milieu toward the Byzantine vision dating to Justinian period. For this reason, the mosaic can be related to pope Sergio I’s (687- 701) commission. Furthermore, the hands of two different artists are recognizable in this decoration. 
The upper church
The basilica was gradually sliding under the ground, together with the rising of the Forum level. Following the works aimed at reclaiming the adverse condition of the basilica, pope Urbano VIII (1623-1644) divided the building in height with a flooring, using the newly obtained upper space as church and transforming pope Felice’s hall in a sort of crypt, now almost deprived of any original embellishment, except for some flooring remains and the ancient altar in pavonazzetto marble (docimenum). Unfortunately, the new flooring of the upper church has caused an altered visual perception of the apsidal mosaics.
Urbano VIII then commissioned the new decoration of the basilica, ordering to raise the band running on the walls, erecting pillars and capitals and enlarging the door that leads to the rotunda from the rectangular building. In remembrance of the transformation brought to the basilica by the pope, a commemorative stone was set under the arch connecting the two spaces. The upper church was built by Luigi Arrigucci, following the previous project by the architect Orazio Torriani. The plan is characteristic of the churches dating to the Counter-Reformation period: two series of side chapels articulated with dividing walls, buttressing the central nave. On each side, three chapels are sumptuously decorated and painted. 
Cappella del S. Crocefisso
In front of the later entrance, one chapel is smaller than the others: it’s the cappella del S. Crocefisso, where the relics of the Saints Cosmas and Damian are venerated, enclosed under the altar, in a pedestal made of red porphyry coming from the inferior church.  The Crucifix is a fresco painting dated to the 13th century and, although later repainted, it represents the unusual iconography of Christ, dressed with a clothing which has been identified with the colobium but, due to the long sleeves, it can be more likely recognized as a tunic. The figure of the crucified Christ dressed with the colobium appears in Rome in a fresco painting at the basilica di Santa Maria Antiqua and in the mostly vanished decoration of the crypt at the basilica di San Nicola in Carcere, documented by a drawing in the Ms. Vat. lat. 5409 (the Christ is depicted with the colobium in the scene of the Flagellation – c. 48r. – and with the tunic in the Crucifixion – c. 45r.). 
Madonna della Salute
The image of the “Vergine della Salute” also called “di San Gregorio” is venerated on the altar. This precious artifact belongs to the Roman school and it was restored in 1914. The Madonna della Salute is one of the rare 13th century altarpiece in Rome, in comparison with the numerous icons created in other Italian regions. Several historians have supposed that the same pope Felice introduced the cult of this Madonna whose earlier image was set between the alabaster altar and the mosaic, according to his will. Ugonio, Poma and Panciroli told that the future pope Gregorio Magno (590-604), when he was still monk, used to visit the ancient image of the Virgin when he passed through the Forum (that icon was substituted with the present one). On the basis of the tradition, when Gregorio was elected as a pope, his visits ceased but one day, while he was passing in the nearby, the Virgin Mary appeared in the same aspect of the sacred image and reproached him. This event has been considered the motivation of the consecration of the central altar in the early basilica. 
The current effigy of the Madonna is painted on a very ancient walnut panel and, according to Matthiae, it can’t be included in the widespread iconographic model of the half-length figure without Child, dating between the 6th and the 10th century AD. This Madonna probably dates instead to the last quarter of the 13th century (ca. 1275-1300) and it was executed by a Roman painter who could know the contemporary artistic expressions, partly embracing some linear decorative characters belonging to the incipient Gothic taste, beside the byzantine conventionalism. Van Marle agreed on the attribution period, noting the unrestrained poses of the figures disposed almost in profile, and he cited other two paintings which can be attributed to the same artist, now both conserved in the United States: one of them, belonging to the Kahn collection in New York, has been considered an artifact by Cavallini according to most critics.
The iconography of this Madonna with Child could be classified among the full-length paintings on panel, even if the panel now appears without the inferior ending. The Madonna is a meaningful document of the artistic exchanges between Tuscany and Rome: its model, iconographic outline and pictorial execution are very close in fact to the Tuscan examples of the late 13th century, yet the Virgin Mary is still seated on a byzantine style throne without the back. Some stylistic and technical aspects also lie outside the Roman framework, as the incisiveness of the drawing and the formal construction of the figure, visible in the way the shapes of the Virgin Mary’s knees emerge under the dress, producing a color lightening. 
Next to the altar, a Cosmatesque candelabra dates to the 13th century. Intended to hold the Easter candle, this spiral column rises on two seated lions. Whitehead and Biasiotti consider this candelabra to be among the few remains of the liturgical ornaments seen by Panvinio inside pope Felice’s ancient church. 
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