The interspinales superiores (spinalis ob liquus ; lombo-sacro-coccygien of Vicq d'Azyr). These muscles are a continuation of the inter spinous muscles of the spine ; but as the spinous processes of the tail are short, and frequently- replaced by two tubercles answer ing to rudiments of the obliqueprocesses, these muscles are here disposed obliquely, being more widely separated posteriorly than they are in front.
The muscles which depress the tail or bend it downwards all take their origin in the in terior of the pelvis, and are prolonged to a greater or less extent along the inferior aspect of the tail. These form, when completely developed, four pairs, or four pairs of series, of muscles.
1. The of Vicq d'Azyr) arise from the internal or pelvic surface of the ossa ilei, and, forming an elong ated fleshy belly in the interior of the pelvis, terminate beneath the root of the tail, which they will consequently depress with consider able force against the anus.
2. The inferior caudien) is the antagonist of the sacro-coccy geus superior, above described, which in struc ture it exactly resembles. This muscle takes its origin from the inferior surface of the sacrum and of the transverse processes of those caudal vertebrm in which these pro cesses are developed, by a fleshy belly which gradually diminishes in thickness, and tcrmi nates by forming as many tendons as there are caudal vertebrm without transverse pro cesses. These tendons are received into sheaths resembling those upon the upper sur face of the tail, and are inserted successively into the base of each caudal vertebra, be ginning about the seventh.
3. The interspinales inferiores (sub-caudales, inter-cocepgeus of Vicq d'Azyr). These are situated beneath the mesian line of the tail. They commence underneath the articulation between the first and second caudal vertebrm, and form an elongated fleshy belly, which, in some quadrupeds that have the tail largely developed, become first of all implanted into V-shaped hones `derived from the fourth, fifth and sixth vertebrm of the tail : they receive, moreover, from time to time additional fleshy slips, which go on continually diminishing in size, and give off' tendons to be inserted suc cessively into the inferior aspect of the base of each caudal vertebra.
4. The pubo-coccygeus (pubo-sous-caudien).
This is a thin rnuscle, derived from the whole extent of the upper margin of the pelvis, and having the appearance of a fleshy membrane. the fibres of which are gradually collected into one point to be inserted beneath the tail into tubercles situated upon the base of the fourth and fifth vertebrm. The action of this muscle will produce an effect similar to that of the ileo-coccygeus.
The muscles adapted to move the tail later ally are arranged in two sets.
1. The ischio coccygeus externus (ischio caudien) arises from the posterior or internal surface of the ischium, a little below and be hind the cotyloid cavity, from which origin it runs backwards to be inserted into the trans verse processes of the anterior caudal vertebrm. This muscle is iinproperly called by Stubbs the levator ani, because in the horse a few fibres of it are connected with the termination of the rectum.
2. The intertransversales (intertransversal of Vicq d'Azyr) extend in the form of inns culo-aponeurotic layers overall the transverse processes that are developed in the caudal region, their tendons of insertion being most distinctly seen upon the upper surface of the tail.
In animals that have the muscular appa ratus of the tail completely developed the muscles are found to consist of eight distinct sets, —viz., two superior, two lateral, and two inferior. In the horse some of these are de ficient, or exist only in a rudimentary con dition. To see them in their full state of de velopment they must be examined in animals provided with long and mobile tails, such as the prehensile-tailed monkey's, the opossums, thelion, and, more especially, in the kangaroo and beaver.
Muscles derir ed fronz the spinal column which serve immediately for the morenzents of the cranium.— These have nearly the same origins as in the human subject, but are comparatively of much greater strength, owing to the inclined position of the head with respect to the ver tebral column. They may be divided into such as proceed, 1st, from the atlas ; 2nd, from the axis; and, 3rd, from the posterior cervical vertebrm and ligamentum nuchm. To the first set belong— ]. The rectus capitis posticus minor, or rather nzedius, arising, as in the human subject, from the atlas ; from this origin it runs to be in serted by a short and broad tendon into the occiput.