In total forty-four horses of different breeds, aged (2 - 31 years) and of both sex (19 mares and 25 geldings), e.g. riding horses, ponies, and Icelandic horses were combined in this study. Forty-one (41) of these horses were dissected to test the hypotheses and isolate and describe the deep lines. Three of the horses were frozen and segmentally sectioned in transverse sections. All the horses were euthanized for reasons unrelated to this study. The horses were either from private owners in a radius of 60 km from Copenhagen City, or in a few cases (approx. 4 - 5) were horses bought from a slaughter house north of Copenhagen for teaching purposes.

Copenhagen University, Faculty of Health & Medical Sciences, Pathobiological Sciences not only approved this project but also provided facilities and monitored its progress.

The deep ventral line is illustrated in Figure 1(a) and Figures 2(a)-(d).

The line corresponding to the human Deep Front Line is named the Deep Ventral Line (DVL) and the description and dissections have been adjusted for the anatomy of the equine quadruped.

From caudal the line has two entrances; one which connects the deep adductor and digital flexor muscles of the hind limbs to the pelvic aperture (see the illustration in Figure 1(a)), and another which connects the ventral compartment of the tail with the lining of the inner cavities. Both parts present connections to the inner organs and the muscles of the pelvis, the abdomen and the thorax to the cranium and the nasal and oral cavities.

The part of the line from the limbs starts at the insertion of the common part of the Tendo m.flexor dig.profundus (tendo communis)at theFacies flexoria phalanx distalis. From here the Tendo communis runs plantar to the Bursa podotrochlearis, the Os sesamoideum distale (the navicular bone), the Phalanx medius and proximalis toOs metatarsale III. At the level of the proximal part of the metatarsus theTendo communis separates into two individual branches which pass medially over the tarsus, Ossa tarsi,heading towards the three capitae of the profound flexor muscle. The muscle bellies originate at the Facies caudalis tibialis and Facies caudalis fibula and Caput fibula. The line continues in a medio-proximal direction towards the stifle where the epimysial lining and fascia from the flexor muscle unites into Fascia genus and theLig.collaterale mediale of the Articulatio femorotibialis. From here it connects into the myofascia of Mm.adductor major et minor and m.pectineus as well as the M.sartorius and into the region connecting to the pelvic and abdominal cavities (Figure 2(a)).

The second part of the line starts in the tail in the Fascia cauda.This fascia forms an envelope around the tail and divides it into four compartments: a dorsal, two lateral and a ventral of which the latter is included in the dorsal part of the DVL. Ventrally to the corpora vertebrae, the fascia cauda continues into M.sacrocaudalis ventralis medialis et lateralis and the M.coccygeus (Figure 2(a)). It then continues into the intrapelvic fascia structures, the adventitia of the most caudal part of the urogenital tract and the most aboral part of the gastrointestinal tract as well as the fascia lining of the intrapelvic vessels and nerves (e.g. nn. pelvini).

From here the line spreads in a cranial direction and includes all the internal linings, the subserosal fascia and all its derivatives related to the abdominal and thoracic cavities, the inner organs, and structures (see Figure 2(b) and Figure 2(c)). Due to the 3-D aspect of this line and so as to provide a better explanation and understanding of this line, subsequent explanation will focus on three pathways 1) the dorsal, 2) the mid, and 3) the ventral.

1) The most dorsal part of the line from the tail and the pelvic cavity continues in a cranial direction to the Spina ischiadica. From here it moves with the Lig.longitudinale ventrale on the ventral surface of the Os sacrum (Figure 2(a)). This ligament follows the ventral surface of the Columna vertebralis until the 7 - 9th thoracic vertebrae where it joins the myofascia of the M.longus colli and the lamina prevertebralis of the Fascia cervicalis. From here it passes through the Apertura thoracis cranialis and into M.longus capitis at the level of C4-C3.Itterminates ventrally at the Basis cranii externa (Figure 2(d)) close to the Synchondrosis spheno-basilaris (SBS) between the Os occipitalis and Os sphenoidale.

2) The second and mid pathway, follows M.sartorius through Lacuna musculorum into the sheet of Fascia iliaca from M. iliopsoas (see Figure 2(a)). It continues in a cranial direction through M.psoas minor and major (Figure 2(a)) to the thoracolumbar junction where it interacts and overlaps with the Crus dexter et sinister of the Diaphragma (Figure 2(b), Figure 2(c))including also the other muscular (pars lumbalis,costalis et sternalis)and tendinous parts of Diaphragma. In the abdominal and pelvic cavity, the line also includes the subserosal fascia of the Peritoneum visceralis aswell as ligaments, plicae and other derivatives related to the urogenital, the mesenterium and the gastrointestinal tract and organs (see Figure 2(c) and Table 1).

The peripheral fascia capsule around the retroperitoneal positioned kidneys and the retropleural positioned heart (the pericardial sac) were found to have the same macroscopic characteristics as the inserting fascia described in humans [13]. These macroscopically identifiable compartments favour the possibility that these organs can move freely yet still remain protected. The insertional fascia layers are also present in close connection to the rest of the abdominal organs and provide the organs with mobility and protection. Generally, the insertional fascia connects the organs to the musculoskeletal system [13]. The connection between the insertional part of the subserosal fascia of the abdominal organs and the locomotion system was clear in several locations. The fascia of the kidneys was found to be closely related to the ventral surface of the vertebrae at the thoracolumbar transition, to the M. psoas major and minor, and to the Fascia transversa of the trunk. The liver fascia connected to the Diaphragma and the M.psoas major and minor and the fascia of the M.transversus abdominis. Moreover, the other abdominal organs such as the pancreas, spleen, gastro-intestinal tract, urogenital tract, ovaria etc. are provided with an insertional fascia first hand and underneath a layer of investing fascia, which are found to be closely related and connected to the organ mesenchyme and the inner organ architecture. An overview of the subserosal insertional fascial connection to the locomotion system is presented in Table 1.

The cranial continuation of this second pathway enters the thorax and on into the subserosal fascia lining of the mediastinum and the pericardium. Collagenous tension lines are present from the base of the heart and in the dorsal mediastinum with a direction pointing towards the mid thoracic vertebrae (Th 7 - 10). Here the line includes the subpleural fascia enclosing the thoracic organs e.g. the esophagus, the trachea, the lungs, and the heart. An intimate contact is observed with the parasympatheticN.vagus, which is situated in the mediastinum from the Apertura thoracis cranialis to the diaphragm. In the closure of the Apertura thoracis cranialis the prevertebral sympathetic ganglions are situated e.g. the Ggl. Stellatum cranially and dorsally under the Costa prima and secunda.

The line continues in the Cavum thoracis through the Apertura thoracis cranialis and follows along the Trachea and Esophagus in the Fascia cervicalis,lamina pretrachealis to the ventral cervical region. The fascia arranges bilaterally into a right and a left Vagina carotis, which include the A.carotis communis,V.cava cranialis,the Truncus vagosympaticus and the N.recurrens.At the Apertura thoracis cranialis the line also comprises M.scalenus from the Costa prima to C4-C7 (the ventral part) and to C7 (the middle part) between which the Plexus brachialis passes through. The right and left Mm.scalenii embrace the trachea. The line continues to thePharynx, Larynx,Diverticulum tubae auditivae and into the profound masticatory muscles of the oral cavity and the tongue (Figure 2(d)).

3) The third and ventral pathway follows the subserosal lining of theLamina parietalis of the walls and ventral surface in the Cavum pelvis,abdominis and thoracis (Figure 2(b)). In the thorax, the subpleural fascia also includes thedense and strongFascia endothoracica.Over the caudal part of the sternum the endothoracic fascia splits and directs dorsally, transforming into the Lig.sternopericardiaca and at the apex of the heart it transforms into the Pericardiumfibrosum. From the thorax in a cranial direction the line continues over the Manubrium sterni and through the M.sternohyoideus to the Apparatus hyoideus and the Articulatio temporo-hyoidea. It also continues rostrally through the hyoid muscles inserting at the rostral part of Mandibula and the structures within the Cavum oris (Figure 2(d)). The two latter pathways unite within theLamina pretrachealis of the Fascia cervicalis.

The function of the DVL is to serve as a flexor and stabilizer of the spine through the hypaxial muscles. The psoas muscles are responsible for the flexion and stabilization of the thoraco-lumbar and the lumbo-sacral region and the longus colli/capitis muscles for the flexion of the cervico-thoracic and the cervico-capital junction. This part of the line also integrates several structural connections of the viscera to the somatic body.

The full line can be divided into three major parts: The two ends (caudal/hindlimb and cranial/neck and head), which include structures mostly related to the somatic part of the body, and the middle part (the abdomen and thorax), which in majority is related to the visceral part of the body.

In contrast to the FLPL and FLRL [1] the deep front limb lines are arranged as slings and comprise several broad aponeurotic fascia sheets related to the scapular and brachial region. The deep lines are focused around the pivot joint in the upper third of the scapula similar to the pro- and retraction front limb lines.

The Deep Front Limb Lines are illustrated in the Figure 1(b), Figure 1(c) and Figures 3(a)-(d).

The FADL starts at the Margo cranialis et Facies cranio-dorsalis scapula with the M.subclavius and M.supraspinatus. The line follows these muscles towards the Sternum accompanied by M.pectoralis descendens and M.pectoralis transversus from Os humeri. Just proximal to the Articulatio humeri at the transitional zone between the scapula and the brachium and on the medial side, is a broad fascia connection which is present between M.subclavius and M.teres major (Figure 3(d)). From here the line proceeds along M.teres major and merges proximally to the Angulus caudalis scapula (Figure 3(a), Figure 3(d)) where the fascia

attaches with tight, strong collagen fibres to both the axial and abaxial side of theScapula. On the medial side, the fascia continues in a dorsal direction intoM.rhomboideus thoracis and in a cranio-transverse direction it continues towards the origin of M.subclavius. The major function of this myofascial line or sling is to direct the front limb into adduction and an external rotation during the last phase of protraction (Figure 1(b)), just before landing occurs on the caudo-lateral part of the hoof. During the landing phase the direction changes into an internal rotation.

The FABL starts dorsal to the scapula inM.trapezius pars cervicalis and thoracalis and a superficial layer of theFascia spinocostotransversus s.Lig.dorso-scapularis. This fascia sheet continues in a distal direction on the abaxial side of the scapula intoM.deltoideus, which surpasses the craniolateral part of theArticulatio humeri towards theTuberositas deltoideus humeri (Figure 3(b)). A fascia sheet continues from here in a medial direction overM.biceps brachii (Figure 3(b), Figure 3(c)). The fascia continues in an axial direction and blends into M.pectoralis ascendens (Figure 3(b)) and proceeds in the sagittal plane into the most cranial part of the aponeurosis of M.latissimus dorsi. From here the myofascial line returns in a dorsal direction into M.trapezius and the Dorsalscapular ligament.The major function of this line is to abduct and internally rotate the front limb during the final phase of retraction and the push off.

The counterpart to the DVL is the Deep Dorsal Line (DDL), which has not been described in humans (Figure 1(a), Figures 4(a)-(d)). The line originates in the coccygeal region in the dorsal muscle compartments of the tail. The M.sacrocaudalis (coccygeus)dorsalis medialis (Figure 4(a), Figure 4(d)), which cranially continues to the Mm.mulitifidi (origo S2/3), and the M.sacrocaudalis dorsalis lateralis, which continues in the space between Mm.multifidi and M.longissimus lumborum at L6, are all parts of the line. In a caudoventral direction at the level of the Tuber ischiadicum the DDL connects to the SDL in two ways: 1) a fibrous contact between the caudal part of M.biceps femoris and the epimysium of M.sacrocaudalis medialis, and 2) a connection betweenM.semitendinosus (endo- and perimysial contact) andM.sacrocaudalis lateralis. In a cranial direction the line joins the myofascia of the transversospinal system e.g. Mm. multifidi (Figure 4(a), Figure 4(b), Figure 4(c)), which span with five fans over one, three and five vertebrae, respectively [39] [40], along the spine until C2. Here it proceeds into the epaxial suboccipital muscles, M.rectus capitis majorand minor andM.obliquus capitis caudalis et cranialis of which the latter attaches additionally to the Crista occipitalis. The suboccipital muscles connect to both the atlanto-occipital and atlanto-axial membranes which again connect to the dura mater, the so-called myodural bridges (Figure 2(b) [41] ). Similar connections are found in the dorsal intervertebral foraminaealong the whole spine.

From the sacral region the line also extends into the intervertebral ligaments and dorsally into the fibrous Lig. supraspinale (Figure 3(c)). It continues in a cranial direction on top of the spinous processes and continues into a transitional zone into a more elastic composition at the level of Th3-Th9, continuing into the Lig.nuchae (Figure 3(b)). The Funiculusnuchae, of the Lig.nuchae, continues cranially and attaches to the Crista occipitalis ventral and medial to M.semispinalis. The Lamina nuchae splits from the funiculi and attaches to the cervical spinous processes (C2 to C5 (C6-7)).

A left as well as a right DDL is present although they run in close proximity. The function of the DDL is to take part in the proprioception and the movement (extension, rotation, and lateral flexion) and stabilization of the vertebrae [14] [39] [42] [43] and thereby the whole spine. The DDL has a close connection to the dura mater both through the myodural bridges but also through the close connection to the Os sacrum [41].

The present study has confirmed the presence of an equine DVL very similar to that of the human Deep Front Line [2]. According to Myers [2] the line comprises three major divisions: two end parts (the legs and the neck) mostly related to the somatic body system as also seen in the superficial lines, and one middle part, a “true 3-D part” (the cavum pelvis, abdominis and thoracis) mostly related to the visceral system of the body.

Several differences between the human (DFL) and equine DVL lines are present and many of them are influenced by the postural difference, biped versus quadruped. One is the inclusion of the M.sartorius in the horse. This connection brings the medial fascia genus in close contact to Mm.psoas and the hypaxial surface of the lumbar vertebrae. The importance of the equine Mm. psoas has been shown by Hyytiäinen et al. [43], who found a muscle fiber composition that indicates that besides flexion of the lumbo-sacral region and flexion of the hip, these muscles act as stabilisers of the lumbar region. The movement or tension from the hind quarter passes directly into the hypaxial compartment of the trunk via the psoas muscles in a cranial direction onto the thoracolumbar junction, where the psoas muscles (origo at the lower thoracic vertebrae) overlap with the diaphragm approaching from a cranial direction (see Figure 2(b), Figure 2(c)). In this region vital structures are situated in close proximity such as the Truncus sympathicus,N.vagus,Ductus thoracicus,Aorta and V.azygos dexter. Biomechanically, the thoraco-lumbar region with the anticlinal vertebrae Th 14-15-16, are vulnerable as the intervertebral movements change gradually from mainly flexion and extension in the lumbar region to include also lateral flexion coupled to axial rotation in the thoracic region [14]. It is at this point that the line changes from being mostly related to somatic/locomotive structures into more visceral.

In a review [44] and in a study with manipulation of the thoracolumbar region Haussler et al. [45] discussed and showed that reduced movement in the lumbosacral junction transmitted the major movement of the spine into the thoracolumbar junction. The biomechanical consequence of such an overload at this junction has to our knowledge not been studied further. Nor have we been able to find research studies on the influence on equine performance, e.g. the respiration, the functionality of the diaphragm, the regulation of the autonomic nervous system, and thereby the regulation of the inner organs etc. When dissecting the profound and the subserosal fascia, the physical tightness between the organs and somatic structures raises questions about the role of the fascia in these viscerosomatic interactions. As presented in Table 1, numerous equine viscerosomatic fascial contacts are present in the line. According to the histological studies [13] clear differences in the composition of the layers of human organ fascia reflect a possible relation to the somatic part of the body. To explore this further there is now a need for more detailed studies of the equine visceral/organ fascia.

The close connection between Mm.psoas and Crus diaphragmatis at the thoraco-lumbar junction makes the diaphragm a very central muscle and not only a divider between the thorax and the abdomen, but also a factor influencing respiration via the somatic body from a distance. Myers [2] explains how the pulling parts of the line in the legs (deep flexor muscles and adductors) are involved in coordinating a rhythm between respiration and locomotion in humans. A feature well known as e.g. in the gallop the push-off and extension of the trunk in the suspensory phase induces an inspiration, whilst landing, the flexion and collection phases induce an expiration [46].

Looked at from another perspective, a respiratory lung problem or rib cage trauma (influencing the function of the diaphragm) could potentially interact with mobility in the hindquarters. The connection between female urogenital or kidney problems to lumbar pain is a frequent condition seen in horses and it makes good sense when studying the subserosal connections to both the hypaxial muscles but also to the fascia connecting to the lateral raphe in the lumbar region.

Minor adaptations in the organ topography are observed with changes in posture, just as well in horses as in humans. In humans, the gastrointestinal tract is connected to the subvertebral regionvia the mesenterium (which in terms of human anatomy is now included as a separate organ/a fascia organ in Grey’s Anatomy [47]. Similarly, in horses the mesenterium attaches the gastrointestinal tract to the dorsum of the abdominal cavity [15] [16].

Denoix and Paillioux [14], illustrates in their book the interplay between viscera and the locomotion system. They explain that the abdomino-pelvic cavity is to be looked upon as an abdominal chamber in which the abdominal muscles contract against the resistance in the chamber created by the diaphragm and internal organs as also [46]. An interaction between the back muscles (SDL and DDL) as well as the abdominal muscles (SVL, LL, SP, FL) and the viscera, balances the forces and pressure both in and on the “abdominal chamber”. This illustration supports our approach of how the superficial lines with their structures outbalance the deep ventral line (DVL).

In the thoracic cavity the lungs in both humans and horses are attached to the mediastinum. The attachment of the heart, in contrast, differs between these two species as does the anatomical posture. In humans the apex of the heart “rests” on the Centrum tendineum of the diaphragm and in the horse on the mid sternebrae. The equine heart, which is situated in a ventrodorsal direction, is attached to the sternum via the sternopericardial ligament including the endothoracic fascia. This fascial structure continues within the pericardial sac to the dorsum of the heart and further dorsally via the dorsal mediastinum to the subvertebral fascial structures of the mid thoracic vertebrae. The embryological development of the pleural serosa and subserosa, the formation and development of the pleura-pericardial folds and the enclosure of the endothoracic fascia can all explain this close connection [48]. Besides the two other ventrodorsal connections (thoracic and pelvic diaphragm) the equine epicardial ventrodorsal connection is the third of this kind present between the dorsum and the venter of the horse.

It might reasonably be argued as to whether Mm. scalenus should be part of the line as described in humans [2], spanning as it does from the first rib to C3 [49]. In horses M.scalenus medialis spans from the costa prima to C7 and M.scalenus ventralis spans from the costa prima to C4 - C7 [16]. Especially the medial scalene does not follow the rule of dissection [2] of multiple joint muscles, however, their close proximity to the Apertura thoracis cranialis and the fascial attachment to the trachea makes it difficult to separate them from the DVL. These muscles are especially important because the plexus brachialis passes between their ventral and medial parts. Thus, should these muscles be kept in a state of static contraction, they could quite easily have a considerable impact on the neurological efferent and afferent output to and from the front extremities.

The Apertura thoracis cranialis is a region in which the wide and complicated part of the 3-dimensional DVL transmits from the thoracic cavity and into a more pulling part of the line through the cervical region. The region is important as the aperture surrounds the en- and extrance of vital organs and connections (included in the DVL), but also because all other myofascial lines (see Table 2) but the FL embrace and outbalance this region. Additionally, the Columna vertebralis is arranged here in a secondary curve, the extension curve,

which compresses the intervertebral joints. Stabilisation and balance in this region are important. This is shown in numerous studies of the effect of head and neck position on the performance and gaits [50] [51] [52] [53] [54], which also comprise the structures in the cranial part of the DVL including the tongue, the pharynx, the larynx, the hyoid, the guttural pouch etc. Additionally, the line attaches the biomechanical collaboration as presented by the stomatognatic system [55] balancing from within, the outer equine superficial myofascial lines (SDL, SVL, LL, SL, FLPL).

The flow of the DVL to the head follows the fascia in the neck and continues ventrally to the cranium, where it attaches to several of the cranial bones which in e.g. the craniosacral and osteopathic techniques are approached to influence the pulsation of the cerebrospinal fluid [56].

Influence on the cerebrospinal pulsation is also assumed to be supported by the MDBs situated in the atlanto-occipital and atlanto-axial regions. These structures are important parts of the DDL, and the pumping mechanism is influenced by the M. rectus capitis major and minor as well as the cranial and caudal oblique capital muscles and the Lig. nuchae, which in the horse is well developed [41].

The DDL balances the DVL around theColumna vertebralis. In humans a Deep Back Line has not been defined [2]. The reason for this according to Myers (personal communication) is due to a lack of connections between Columna vertebralis and the hindlimb. Although, these connections are clearly visible in the horse as the myofascia of the coccygeal muscles are connected to the myofascia from M.biceps femoris andM.semitendinosus, both relate to the SDL and biceps also to the SL. In a cranial direction the line includes the longitudinal spinocostotransversal system of theColumna vertebralis and has an influence on proprioception, balance and mobility of the vertebrae as well as strengthening this part of the core system. Indeed, in support of this function, lumbar pain has been found to be associated with a reduction in the cross-sectional area of the M.multifidi [39] [40] [57] [58].

From the tail and in a cranial direction the Lig.supraspinosus is the most dorsal part of the DDL. The ligament conjoins the movements of the vertebrae. It is fibrous until the lower part of the cervicothoracic junction where it transforms into the Lig.nuchae. This ligament supports the cervical vertebrae and the cranium in locomotion, especially in relation to the oscillation of the head during walking. In modern horse breeds the laminae nuchae reach from C2 to C5 [59], whereas in many anatomical descriptions (perhaps of older date) the laminae attach to all the cervicals up to C6-C7 [15] [16]. The lack of the lower laminae nuchae increases the vulnerability of the cervicothoracic region as also mentioned previously.

The armlines (human) and front limb lines (horses) are influenced by not only a difference in posture but also by biomechanics. In the quadruped horse a vertical line of balance goes from the proximal third part of the scapula to the mid region of the hoof. This line divides the front limb into a protraction and a retraction compartment [1]. The line of balance is essential in the mechanical stay apparatus of the front limb, where it among other things secures the horse, enabling it to stand and rest in balance with only minor energy consumption and without overuse of the anatomical structures. In the brachium, antebrachium and the foot the FLPL is mostly represented dorsally by the extensor muscles and tendons. The flexor muscles and tendons represent the FLRL. A cross-over of the pro- and retraction lines at a point close to the top third of the scapula indicates the location of the pivot joint, the thoraco-scapular joint. This joint is the centre of motion in the front limb and the top point for the line of balance as mentioned above. The thoracic sling which is composed of only muscular and tendinous attachments between the front limb and the thorax supports free and flexible movements including both pro- and retraction, but also and in majority in this region facilitates ad- and abduction as well as in- and outward rotation, thereby providing a functional reason for the anatomical location of the deep front limb lines.

The equine deep front limb lines connect to numerous other structures and lines such as: M. rhomboideus (SP, FLPL and FLRL), M. trapezius (FLPL, FLRL), M. serratus ventralis (SL), M. latissimus dorsi (FL), M. pectoralis ascendens (FLRL, FL) (see Table 3). Thus, in situations where there is a change in motion pattern in the front limbs, compensatory motion can affect other lines through these connections, contributing to imbalance. In this way it is easy to understand how front limb imbalance can interfere with the posture of the rest of the body and vice versa. Indeed, just such an interaction could explain the compensatory

weight shift in the body and towards the front limbs that occurs with changes in head posture. Moreover, a reduction in the mobility of the lumbosacral junction results in a reduction in the motion of the lumbar region whilst increasing a translocation of movement in cranial direction towards the front part of the horse [44].

So, stepping back for just a moment and considering what has been presented in this study, it is important to re-visualize the myofascial lines as being arranged in the form of a 3D skeleton, which spans the entire body, and serves to counteract changes in tension whilst attempting to balance the body, according to the tensegrity model, while maintaining a new posture or facilitating motion. Of course, under normal conditions this is how the myofascial lines interact, enabling parts of the lines to move freely in relation to other lines or structures at all times.