Fig. 240. - A, Lumbar vertebra of a Whale : c Body or centrum; n n Neural arches ; s Neural spine; a a Articular processes; d d Transverse processes. B, Diagram of a thoracic vertebra: c Centrum; n n Neural arches enclosing the neural canal; s Neural spine ; r r Ribs, assisting in the formation of the haemal arch; p p Costal cartilages; b Sternum, with haemal spine. (After Owen.)
The haemal arch in the case of the human thorax (fig. 240, B) is formed by the ribs (r r) and the costal cartilages (p p), and is completed in front by the breast-bone or sternum (b), which in some cases - but not in man - develops a spine (the haemal spine) which corresponds to the neural spine on the opposite aspect of the vertebra.
It follows from the above, that the typical vertebra consists of a central piece or body from which two arches are given off, one of which protects the great masses of the nervous system, and is therefore said to be "neural;" whilst the other protects the main organs of the circulation, and is therefore said to be "haemal." The correspondence of the typical bony segment or vertebra with the doubly tubular structure of the body in all Vertebrates is thus too obvious to require to be specially pointed out.
As a general rule, the vertebral column is divisible into a number of distinct regions, of which the following are recognisable in man and in the higher Vertebrata: 1. A series of vertebrae which compose the neck, and constitute the "cervical region" of the spine (fig. 241, n). 2. A number of vertebrae which usually carry well-developed ribs, and form the "dorsal region" (d). 3. A series of vertebrae which form the region of the loins, or "lumbar region"(l). 4. A greater or less number of vertebrae Which constitute the "sacral region," and are usually amalgamated or "anchylosed" together to form a single bone, the "sacrum." 5. The spinal column is completed by a variable number of vertebrae which constitute the "caudal" region, or tail (c).
As regards the skull of the Vertebrata, it has been thought advisable not to enter into any general details here, partly because the subject is one which can only be properly discussed in a work specially devoted to Human or Comparative Anatomy, and partly because there is still much diversity of opinion as to the exact composition of the skull. There is, however, a very general concurrence of opinion that the skull is composed of a series of separate segments, and this is a point which it is important to remember. By Owen, and by many other competent authorities, these cranial segments are looked upon as being nothing more than so many vertebrae, the neural canals of which are greatly expanded to enclose the brain, whilst the haemal arches are very greatly modified to serve different purposes. This view is not accepted by high authorities; but the general fact that the skull is composed of separate segments is universally admitted. The only portion of the bony framework of the head which it is absolutely essential to understand, is the lower jaw or "mandible." The lower jaw is sometimes wanting, but when present, it consists in all Vertebrata of two halves or "rami," which are united to one another in front, and articulate separately with the skull behind. In many cases, each half, or "ramus," of the lower jaw consists of several pieces united to one another by sutures; but in the Mammalia each ramus consists of no more than a single piece. The two rami are very variously connected with one another, being sometimes only joined by ligaments and muscles, sometimes united by cartilage or by bony suture, and sometimes fused or anchylosed with one another, so as to leave no evidence of their true composition. The mode by which each ramus of the lower jaw articulates with the skull also varies. In the Mammalia the lower jaw articulates with a cavity formed on what is known to human anatomists as the temporal bone; but in Birds and Reptiles the lower jaw articulates with the skull, not directly, but by the intervention of a special bone, known as the "quadrate bone" or os quadratum.
Fig. 241. - Skeleton of an Armadillo, showing the regions of the vertebral column. c Cervical region; d Dorsal region ; / Lumbar region; s Sacral region; t Caudal region or tail.
As regards the limbs of Vertebrates, whilst many differences exist, which will be afterwards noticed, there is a general agreement in the parts of which they are composed. As a rule, each pair of limbs is joined to the trunk by means of a series of bones which also correspond to one another in general structure. The fore-limbs, often called the "pectoral" limbs, are united with the trunk by means of a bony arch, which is called the "pectoral" or "scapular" arch; whilst the hind-limbs are similarly connected with the trunk by means of the "pelvic arch." In giving a general description of the parts which compose the limbs and their supporting arches, it will be best to take the case of a Mammal, and the departures from this type will then be readily recognised.
The pectoral or scapular arch consists usually of three bones, the "scapula" or shoulder-blade, the "coracoid," and the "clavicle" or collar-bone; but in the great majority of the Mammals, the coracoid is anchylosed with the scapula, of which it forms a mere process. The scapula or shoulder-blade (fig. 242, s) is usually placed outside the ribs, and it forms, either alone or in conjunction with the coracoidal element of the shoulder-girdle, the cavity with which the upper arm is articulated. The coracoid, though rarely existing as a distinct bone in the Mammals, plays a very important part in other Vertebrates, as we shall see hereafter. The clavicles are often wanting or rudimentary, and they are the least essential elements of the scapular arch. The fore-limb proper consists, firstly, of a single bone which forms the upper arm (or "brachium"), and which is known as the humerus (h). This articulates above with the shoulder-girdle, and is followed below by the fore-arm (or "antibrachium"), which consists of two bones called the radius and ulna. Of these the radius is chiefly concerned with carrying the hand (or "manus"). . The radius and ulna are followed by the bones of the wrist, which are usually composed of several bones, and constitute what is called the carpus (d). These support the bones of the root of the hand, which vary in number, but are always more or less cylindrical in shape. They constitute what is called the metacarpus. The bones of the metacarpus carry the digits, which also vary in number, but are composed each of from two to three cylindrical bones, which are known as the phalanges (p). Homologous parts are, as a rule, readibly recognisable in the hind-limb. The pelvic arch, by which the hind-limb is united with the trunk, consists of three pieces - the ilium, ischium, and pubes - which are usually anchylosed together, and form conjointly what is known as the innominate done (fig. 243, i). In most Mammals, the two innominate bones unite in front by ligamentous or cartilaginous union, and they constitute, with the sacrum, what is known as the pelvis. The hind-limb proper consists of the following parts: - 1. The thigh - bone or femur, corresponding with the humerus in the fore-limb. 2. The bones of the shank (or "cms"), corresponding with the radius and ulna of the fore-limb, and known as the tibia and fibula. Of these, the tibia is mainly or altogether concerned in carrying the foot (or "pes"), and it is thus shown to correspond to the radius, whilst the fibula corresponds to the ulna. 3. The small bones of the ankle, known as the tarsus, and varying in number in different cases. 4. A variable number of cylindrical bones (normally five), which are called the metatarsus, and which correspond to the metacarpus. 5. Lastly, the metatarsus carries the digits, which consist of from two to three small bones or phalanges, as in the fore-limb.