ANATOMY the higher vertebrae the coelom or body cavity which they line is formed by a splitting of the mesoblast, the comparative embryology of this cavity shows that originally in the most archaic vertebrates it was formed by the outgrowth of diverticula from the primitive gastrieal cavity. It is consequently possible to sustain the thesis that the apparent delamination in the higher forms is due to the superposition of a series of processes of development which occur successively in the lower, but simultaneously in the higher vertebrate. Hence it is conceivable that the lining material of the coelom may be not an endothelium, but a true epithelium. /The same may possibly be the case in the great blood-vessels. At the same time it must be remembered that it is not possible to draw hard-and-fast lines between the derivatives of the several germ layers, whose distinction the one from the other is not so definite as was once imagined. Serous membrane is always lined by a continuous layer of pavement cells, resembling pavement epithelium in almost every respect, and the same is the case with the blood and lymph vessels. Splanchnology.
397
of bone which project beneath the skin. To this the name deep fascia is given. Partitions of this material dip in between groups of muscles, and are called intei muscular septa. Other lamellae form sheaths for vessels, nerves, and viscera. The deeper connective tissues forming the main axis of the body, to which the name skeleton is commonly limited, are composed of bone, cartilage, and fibrous tissues, and are disposed segmentally. They consist (1) of the vertebral column, and (2) of the skeletal walls of the two great tubes of which the vertebrate body is made up, the dorsal or neural, and the ventral or visceral. The walls of these tubes are united to the investing layers of deep fascia by four longitudinal partitions—one mediodorsal, one medioventral, and two lateral subdividing the body wall into four areas, two dorsolateral and two ventrolateral. At their first appearance in the embryo the continuity and segmental arrangement of these connective masses .and partitions is obvious ; but in their later conditions, when parts of this skeletal apparatus are transformed into cartilage and bone, though to some extent appearances of segmentation still persist, yet these are often of derivative origin, and do not coincide with the primary segmental divisions. For in the course of development the primary intersegmental clefts are in part seen to lade and a gegment. secondary series to appear ; these, instead of coincidins, are for the most part alternate with them. . Inis secondary segmentation occurs especially in the tissue around the notochord or unsegmented axial rod (vol.q. p. 830). This notochord is no part of the skeleton ; it was originally a canal, but in the absence of any animal which preserves this structure as such it is difficult to understand what was its original function (tig. o). Epiblast. Notochord.
A mass made up of various tissues united so as collectively to discharge some special function is called an wgan. Proximately the human body consists of an assemblage of organs arranged in a definite manner, and the study of the structure of these organs and of their relative positions constitutes the subject-matter of. Descriptive Anatomy. Organs may be classified physiologically according to their respective offices into seven groups (1) those of °r8ans- protection and support (skeletal) ; (2) of motion (muscles): (3) of sensation, the seats of the reactions dependent on and related to sensation and psychic process (nervous) • (4) those whereby food is taken in and assimilated (digestive) ; (5) those whereby the nutritive material is distributed to the tissues (circulating) ; (6) those whereby Avaste products are got rid of (eliminative) ; and (7). those by which reproduction takes place. Morphologically, organs may be divided into two groups, according to their relation to the originally segmented constitution of the body The human body, like that of all vertebrate animals, is made up of a chain of segments, each of which is more or less built on a common plan. The organs of the 1st, 2nd, 3rd, 5th, and some of those of the 6th group are segmental in original plan, although in the course o development this characteristic has become obscured Those of the 4th and 7th, and some of those of the 6th group show no sign of ever having been segmented at any stage in their ontogeny. . I Organs of Protection and Support.—All the active tissues of the body are supported on a framework of connective tissues, or else are contained m spaces bounde< y lamellse of these tissues, which thus constitute the skeleton of the organism. (The word skeleton is populariy limited to the bony framework, but, morphologically, all the connective tissues are equally parts of the vertebrate skeleton.) A layer of extremely condensed areolar. tissue invests th whole body, forming the true skin, cutis ox dermis. T is is truly skeletal, and in some lower vertebrates is the on y tissue in which ossification occurs. Beneath this is a laxer layer, also of areolar tissue, differing from the last chiefly in its lesser density, in its containing more or less tat m its meshes, and in its being ^ nerves, which do not end m it as they do m the denser cutis which invests it. This layer is called ™perfic d fascia, and, like the skin, it is continuous over the whole
pm fi Diaaram constructed from several sections of the vertebral region of embryo^arkthe“wing the formation vertebral bodies («.6.)c^ around ttetLnotocho'rdrby rearrangement of theofscleroblasts or inner a of the original protovertebra, the lines of segmentation of these rearranged segments being alternate with those of the original P^^ra, as shown between the muscle plates (ro.p.) and cutis plates (ewh), from which the muscular system and dermis are respectively developed. The epiblast gives rise to the cuticle. Between the muscle layer and the cutis layer is the segmented portion of the ctelom. It probably was an archaic form of alimentary canal, aiound which were disposed the segmented skeletal and other tissues. The connective tissues which surround it become the successive vertebral bodies, and their uniting fibrous discs, from the former of which the neural arches project dorsally, and the costal or nb arches
To the description of the vertebral column in vol. i. P- 821 it is only necessary to add that the morphological, relationships ol the parts in successive vertebrae are not so simple yertebrae. as they appear at first sight. Each vertebral body . originates as two separate elements — one a per ichor dal ling, arising in two symmetrical parts, one on each side of the notochord, and a second or hypochordal arch connected with t bases of the neural and visceral arches, lying on the yentia of the perichordal elements and on a level with the ™terva between contiguous perichordal rings This hypochordai the ele merges with the perichordal in all vertebra, except nrst b °B^neath the superficial fascia there is in most parts of cervical. In this it remains separate and ossifies as the anterior arch of the atlas, while the corresponding perichordal rm^, bethe body a firm, thin layer of white fibrillar tissue,, whic comes the odontoid process of the axis. The successive vertebra covers the muscles and is attached to those prominences
