Leonardo da Vinci was one of the most important renaissance personalities of his time, and the fifth centenary of his death will be commemorated in 2019. Being the illegitimate son of a notary, he did not continue the family saga and was educated privately. He had no formal education, thereby not conditioning his curiosity about the world around him. The erudite texts of his time were written in Latin and Greek, languages he did not master, and his access to the literature was therefore limited.

He was an artist and a scientist. As a painter, scientist, engineer and theorist, he produced thousands of drawings^[1], personifying the 'Renaissance man' skilled and versed in arts and sciences^[2].

His interest in anatomy was overwhelming, proven by the numerous sheets dedicated to his anatomical studies, with abundant notes and drawings, exemplifying Leonardo's principle that anatomic parts and organs should be represented in multiple views. Considering that dissections of human corpses outside Universities were not regarded as appropriate by the ecclesiastical authorities, he performed some dissections of animals. According to the Royal Collection Trust^[3], at the outset of Leonardo's anatomical investigations, he was unable to procure much human material. Hence, many of his dissections were therefore of animals.

Practically his entire collection of anatomical drawings was compiled in the Windsor Codex, property of Her Majesty Queen Elizabeth II. These drawings of the human body were exhibited in an unprecedented exhibition in 2012 at the Queen's Gallery, Buckingham Palace (London, UK). Although previous access to the collection was highly restricted, nowadays, the Royal Collection Trust offers the possibility of free access to these drawings in high resolution on its website, which greatly enables the observation of these masterpieces and their details.

Several works have been published based on these anatomical drawings, the most exhaustive ones are those from the collection of three volumes from Clark^[4][5], compiling all the inventory information, the book from O'Malley and Saunders^[6] and its posterior editions in 1983 and 2003^[7][8], and the official book of the exhibition. Clayton and Philo^[9] and another book published in 2013^[10], the two latter reviews, mainly referred to human anatomy, although they also include comments on some animal anatomy drawings. Apart from books, there are numerous scientific articles sharing the same subject: Leonardo da Vinci's anatomical drawings, mainly intended to some areas of expertise, such as those from Schultheiss et al.^[11], Jose^[12], Ganseman and Broos^[13], Pasipoularides^[2], Sterpetti^[14], Bowen et al.^[15] and West^[16], among others.

It is well known that Leonardo dissected numerous animals^[17]. As a result, many endeavors have been made to identify the animal of which the individual anatomical drawings have been made. In some cases, such identification is easy, while in others it is impossible^[17]. Leonardo da Vinci's methods of acquiring knowledge were observation and experiment, and for him, the study of anatomy became a science, combining both the study of structure and function^[12].

Reviewing the work of several authors on the description of Leonardo's animal anatomy drawings, and comparing them with the high-resolution images available on the website of the Royal Collection Trust^[3], it can be noted that some of them were not properly described elsewhere, with some inaccuracies or misunderstandings that deserve to be discussed, probably due to the fact that the consulted authors were experts in human anatomy and, therefore, had no deep understanding of animal anatomy. Hence, it is important to point out that human anatomy could be considered similar, although with some differences, to animal anatomy. For major details, all of Leonardo da Vinci´s anatomical drawings can be accessed on the Royal Collection Trust website^[3].

Regarding Leonardo's anatomical drawings, apart from human anatomy, he also depicted some animal species such as dogs, bears, pigs, horses, oxen and monkeys. The main aim of this comparative study on anatomy was focused only on two species: bears and horses.

According to Forlani-Tempesti^[1], da Vinci mentioned bears in his notes for his anatomical treatise: “I will discourse of the hands of each animal to show in what they vary; as in the bear which has the ligatures of the toes joined above the instep”, and again: “Here is to be depicted the foot of the bear or ape or other animals to show how they vary from the foot of man or, say, the feet of certain birds” ^[1]. Bears are also the protagonists of other drawings of da Vinci: A bear walking (Robert Lehman Collection) and three other studies of a bear's (or a wolf's or dog's) paws (1490–1495) and head (Colville Collection)^[1]. However, those images of paws cannot be from a bear, simply because bears have a manus (hand) with five digits (fingers) with their distal phalanxes (claws) in contact with the floor. In contrast, a dog and wolf manus only have four digits ending in unguicula (claws) in contact to the ground, plus the digit I (dewclaw) medially placed at the level of the metacarpal bones.

2. Flaws of the Anatomy of the Bear's Pes (Foot) and the Hidden Antebrachium (Forearm) and Manus from a Dog/Wolf

Later on, according to Clayton and Philo^[10] “Writing in the mid-sixteenth century, the biographer Giorgio Vasari stated that Leonardo compiled a treatise on the anatomy of the horse. One drawing of the viscera of a large quadruped, probably a horse, does survive from this period, suggesting that Leonardo conducted full dissections to investigate the internal anatomy of the beast. But Vasari also stated that the treatise on the horse was lost when Milan was invaded by French forces in 1499. Ludovico Sforza was overthrown, and soon afterwards, Leonardo left the city and returned to Florence” ^[10]. This text refers to the drawing RCIN 919097-recto, entitled ‘The viscera of a horse' (1490–1492; Figure 2), and described at the Royal Collection Trust^[3] as: “an anterior view of the arteries, veins and the genito-urinary system of an animal, probably a horse,” implying that Leonardo did not name this drawing. The drawing represents the ventral aspect of the trunk of an animal (supposedly, a horse) with the lungs and the canalis alimentarius (esophagus, stomach and intestines) removed. The large vessels depicted at the centre, all the figure down, represent the aorta (on the right of the figure) and the vena cava caudalis (on the left of the image). The way they ramify helps us to discard the idea that this drawing represents a human being. In humans, the aorta ends up dividing into two branches: Aa. iliaca commune (dextra and sinistra). In contrast, in animals, the end of the aorta abdominalis (at the level of the pelvic limbs) produces two branches (external iliac arteries—dextra and sinistra), well depicted, and subsequently continues and produces two more (internal iliac arteries), well represented in the drawing, ending as the arteria sacral median, not depicted. Regarding the veins, the vena cava caudalis is formed by the confluence of two Vv. Iliaca commune—dextra and sinistra, each one resulting from the junction of the V. iliaca externa and the V. iliaca interna, following a similar pattern both in humans and domestic mammals.

Regarding the blood vessels, the drawing (Figure 2) provides three key points: (a) The first huge vessel (on the left of the image), reaching the heart, could be the Vena cava cranealis, and the other curved vessel going down is the aorta and its arcus aortae, with two big arteries leaving the aortic arch and some smaller ones (2–3) once the arch finishes and continues to the descendent aorta (aorta descendens). Large domestic mammals (horses—Eq, and ruminants—Ru) only have one artery deriving from the aortic arch, the truncus brachiocephalicus, which is then divided into a truncus bicaroticus (could be absent in carnivores—dogs and cats) and two Aa. subclaviae. In contrast, carnivores and pigs (Su) have two arteries leaving the arcus aortae: The truncus brachiocephalicus first and secondly the A. subclavia sinistra. (b) On the other hand, at the kidney level, there are two arteries perfectly outlined in the drawing stemming from the aorta: The A. circumflexa ilium profunda (dextra and sinistra), exclusive to carnivores ^[21] and dividing into the rami craniales and caudales. In contrast, the Aa. circumflexa ilium profunda derives from the A. iliaca externa in Su, Ru and Eq ^[21], similar to humans^[22], not stemming directly from the aorta. (c) The arteria and vena circumflexa ilium superficiales, the first branches of the A. femoralis and V. femoralis, respectively, are exclusive to carnivores ^[21]. They leave their main vessels cranially oriented, at the medial and proximal part of the thigh. These vessels (a–c) in this drawing are the main clue to determine the species. Consequently, the horse representation/provenance of this drawing could be discarded. However, the horse is the unique domestic species in which the aorta does not end caudally as an arteria sacral median, which is not represented in the illustration.

Thanks given to the Royal Collection Trust for allowing free access to their digital collection of Leonardo da Vinci's legacy, and for granting the copyright of these images free of charge.^[22]