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Revista chilena de anatomía

versión impresa ISSN 0716-9868

Rev. chil. anat. v.17 n.2 Temuco 



Arruda, O. S.
Arruda, M. S. P.

Department of Biological Sciences, State University of Sao Paulo, UNESP, Bauru, SP, Brazil.
This paper was supported by CAPES Grant

SUMMARY: The vessel bundle formed by the ramification of the median sacral artery was studied in 20 armadillos (Dasypus novemcinctus). This bundle, considered to be a rete mirabile, is located in the sacral region close to the spine and to the base of the tail. The parallel arrangement of its arteries and veins gives origin to a countercurrent system that probably prevents heat dissipation through the end of the tail, thus participating in the control of body temperature in the armadillo.

KEY WORDS: 1. Anatomy; 2. Artery; 3. Abdominal aorta; 4. Dasypus novemcinctus.


Since the report by KIRCHHEIMER & STORRS (1971), which led to a more intensified use of the armadillo in medical-biological research, several problems have accumulated due to the lack of information about the biological characteristics of this animal. Hypothermia represents an important aspect, especially because it originated the leprosy studies. The objective of the present study was to provide information for the clarification of some aspects related to the mechanisms of temperature control by an anatomical study of the ramifications of the median sacral artery previously reported by ARRUDA & NAVARRO (1998). Some anatomical aspects of each armadillo studied were described in order to provide support for these studies.


Twenty adult armadillos (Dasypus novemcinctus) of both sexes native to the central region of the state of São Paulo, Brazil, were provided by the animal house of the

Lauro Souza Lima Institute and Institute of Biosciences (IBAMA registration Nº 015113, license N° 246/96). The animals had been destines to experimental studies on leprosy and paracoccidioidomycosis but did not survive the period of adaptation to captivity. The armadillos were utilized experimentally according to the recomendations of COBEA (Brazilian College of Animal Experimentation). For the macroscopic studies, neoprene latex was injected into the arterial system of 15 animals which were then fixed in 10% formalin. The animals were submitted to macroscopic dissection and dissection under the stereoscopic microscope at 12X to 16X magnification. The median sacral artery and its ramifications were observed after retraction of the skin and of the subcutaneous layer of the abdominal surface and removal of intestinal loops, colon, rectum and rectal ampulla. Fragments of the ramification of the median sacral artery were removed from 5 newly sacrificed animals for histology. The fragments were routinely processed by dehydration, clearing and embedding, and 6 to 8 µm sections were obtained and stained by the Masson trichrome and Calleja methods.


Vessel bundle: the median sacral artery behaved as a continuation of the abdominal aorta, emerging between the internal iliac arteries with a caliber similar to that of the aorta. After running ventrally with respect to the vertebral bodies for 1 to 15 mm, the artery abruptly emitted a series of vessels that continued along this trajectory, forming a vessel bundle of oval conformation measuring approximately 70 mm in length (Figs. 1 and 2). Examination of transverse sections revealed the relationship between these arteries and veins as a regular distribution of vessels running side by side (Fig. 4). This interrelationship was constant throughout the extension of the bundle. The small magnification provided by the stereoscopic microscope revealed the presence of various arterial and venous anastomoses, although no arteriovenous anastomoses were observed. Several nerves and ganglia were detected in the structure of the vessel bundle.

Superficial anatomy of the tail: the tail of the animals studied measured approximately 40 cm in length. In the region where the tail joined the trunk there was a circular area approximately 8 mm in extension that was devoid of bony lining. Starting from this site, the tail was fully covered with bony dermal plates initially arranged into concentric articulated rings that lost individuality as they approached the extremity (Fig. 3).

In the cranial third of the tail, the conical rings consisted of three rows of perfectly joined bony plates, two pentagonal ones intercalated with a hexagonal one. These tended to form a rectangle as the rings decreased in diameter. In the final third the rings disappeared completely, the plates articulated with one another acquiring a diamond shape and maintained this aspect to the end of the tail.

In the cranial portion the tail exhibited a dorsoventrally flattened oval shape measuring about 8 cm in its largest diameter. The tail then became almost cylindrical and its diameter was gradually reduced until the tip, where it was reduced to only 1 mm.

The ventral part of the tail showed poorly pigmented bony dermal plates, a fact that conferred a whitish coloring on this region. The lateral and dorsal parts consisted of bony plates rich in black pigment that gave these parts of the tail a color identical to that of the entire body of the animal.


According to FAWCETT (1942), the term rete mirabile was first used by Galen and Willis, among others, to designate the carotid network which consists of a specific vessel bundle formed during the course of the artery. In these cases the artery loses its identity, with the formation of a mesh of vessels that later coalesce an lead again to the formation of one or more arteries. This definition of rete mirabile has also been used by ROMER & PARSON (1985). Again according to FAWCETT, a large variety of arrangements of blood vessels, more or less plexiform, has been recognized macroscopically in the three major orders of aquatic mammals, i.e., Cetacea, Pinnipedia and Sirenia, as well as in some members of Edentata and Lemurs. In general, these arrangements have been called rete mirabile.

Indeed, DAVIES et al. (1947) reported that MULLER (1904) had already noted the multiplicity of the morphological aspects of these vascular plexuses, which WISLOCKI & STRAUS (1932) identified as a vascular network and bundle, according to the morphological characteristics observed. The descriptions made by these investigators of the bundles detected in the limbs of sloths suggest a similarity to what was observed in the ramification of the median sacral artery of the armadillo, an animal that, like the sloth, belongs to the order Edentata. Within this variety of morphological aspects and arrangements, the vascular bundle that surrounds the median sacral artery of the armadillo can be definitely included among the formations that have been called rete mirabile.

The anastomoses observed in the vessels that formed the bundle detected in the armadillo were, according to POIRER et al. (1908), of the simple transverse type and morphologically and quantitatively similar to those detected by DAVIES in the pelvic member of the loris (Nycticebus tardigradus malaianus).

SCHOLANDER & KROG (1957), SCHIMIDT-NIELSEN (1972) and ROMER & PARSONS have stated that the rete mirabile is an anatomical arrangement whose function is to prevent loss of heat. The countercurrent system formed by the arteries and veins arranged in parallel causes the heated arterial blood to transfer heat to the venous blood that returns from the limbs. Thus, this mechanism prevents heat from reaching the extremities where it would be dissipated into the environment.

Fig. 1 Lateral view of the ramification (rete mirabile) of the median sacral artery (arrowheads). X 2.5.

Fig. 2 Ventral view of the abdominal aorta (a) and its branches: external iliac artery (b); internal iliac artery (c); median sacral artery (d).

Fig. 3 Tail. Note the arrangement, morphology and color of the bony dermal plates (arrowheads).

Fig. 4 Transverse section of the vessel bundle (rete mirabile). Arrangement of arteries (asterisk) and veins (arrowheads). X 50.

The arrangement described, consisting of the branches of the median sacral artery of the armadillo, was located close to the sacral vertebrae and its end occurred close to the beginning of the caudal vertebrae. On this basis, the countercurrent mechanisms may act by preventing the heat conducted by the median sacral artery from fully reaching the animal's tail, since the latter seems to play an important role in this system, as a function of some of its characteristics described below.

The tail of the armadillo, when considered as a whole from the beginning of the first articulated ring to the tip, was approximately 40 cm long, corresponding to approximately the length of the rest of the animal's body. On the other hand, the rigid aspect of the bony dermal plates and the way they are arranged to form the cover give little mobility to the animal's tail.

Since this is an animal of short stature, approximately 20 cm, its tail is almost fully in contact with the soil surface when the animal walks. The tail also contacts the soil surface when the animal uses it as support to elevate the thoracic limbs or as a guide for orientation towards its burrow when the animal carries material to build its nest (STORRS, 1971).

Taking into account the above characteristics, which point out important factors such as constitution of the tegument, dimensions, morphology and function, it is acceptable to conclude that the tail of the armadillo may act as a heat-dissipating element. Thus, the interaction between the tail and the vessel bundle that surrounds the median sacral artery may be responsible for an important temperature-regulating mechanism.

Behavioral aspects of the armadillo may contribute to the justification of the presence of such morphological arrangements in these vessels.

According to STORRS, this is an animal of nocturnal habits that remains sheltered in burrows during the day. TALMAGE & BUCHANAN (1954) reported a wide distribution of these animals from Argentina to the United States. On this basis, we may deduce that this animal species survives in highly distinct climatic regions and is exposed to great variations in temperature. In support of this statement are also the hypothermia in relation to other mammals and the broad spectrum of variation of the armadillo's body temperature, which some investigators consider to be 30oC to 36oC (ALMEIDA & FIALHO, 1924; EISENTRAUT, 1932; WISLOCKI & ENDERS, 1935; ENDERS & DAVIES, 1936; JOHANSEN, 1961; BURNS & WALDRIP, 1971; MERCER & HAMMEL, 1989). These characteristics definitely require special temperature-regulating mechanisms that may be related to the structures discussed here.

Reports by SCHOLANDER & KROG about animals possessing the rete mirabile have compared fast marine mammals to slow-moving terrestrial mammals. These investigators described the characteristics of the animals studied and of the environments where they lived and suggested that the development of the rete mirabile may have occurred during the evolutionary process by adaptive convergence in response to some determining factor. It seems acceptable to infer that aspects related to temperature control may have played an important role in the development of these processes, as supported by TALMAGE & BUCHANAN in their descriptions of the different climatic regions inhabited by the species Dasypus novemcinctus, and by BERNARD (1876), cited by SCHOLANDER & KROG, in his observations about the heat-retaining functions of the rete mirabile.

Obviously, more in-depth studies are needed to establish perfect correlations between this structures and the mechanisms in question, but the strategic location of the rete mirabile, allied to the clear needs of the animal, validates these assumptions. Investigations along these lines are currently underway to better characterize these structures and to obtain data that will permit to clarify other aspects involved.

RESUMEN: El conjunto de vasos formados por la ramificación de la arteria sacral mediana fue estudiado en 20 armadillos (Dasypus novemcinctus). Este conjunto, que ha sido considerado una rete mirabile, se extiende desde la región sacral de la columna vertebral hasta la base de la cola. La conformación de las arterias y sus venas paralelas da origen a un sistema de contra-corriente que seguramente previene de la disipación de calor a través de la cola y, de esta manera, contribuye con los mecanismos del control de la temperatura corporal del armadillo.

PALABRAS CLAVE: 1. Anatomía; 2. Arterias; 3. Aorta abdominal; 4. Dasypus novemcinctus.


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Dirección para correspondencia:
Prof. Dr. Olavo Speranza de Arruda
Department of Biological Sciences, State University of Sao Paulo, UNESP,
Av. Luiz Edmundo Coube, sn. CEP 17033-360, Bauru, SP,
Tel +55-14-2302111, Fax +55-14-2306987,

Recibido : 15-03-1999
Aceptado: 28-09-1999

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